Senate Democrats opened the door to multiple bites at the budget reconciliation apple this year and next after parliamentarian guidance earlier this week said revisions to previously adopted budgets could also trigger more filibuster-proof legislative packages.

But the bare-bones advisory from Senate Parliamentarian Elizabeth MacDonough said simply that reconciliation instructions “may” be allowed in a revised budget resolution under Section 304 of the 1974 budget law.

It didn’t say anything about the types of reconciliation bills that could be considered; how many times the maneuver would be allowed; or whether the fiscal 2021 budget resolution could be amended to “reset” its reconciliation instructions – previously used for the $1.9 trillion COVID-19 aid package – for something like President Joe Biden’s infrastructure plan.

Senate Majority Leader Charles E. Schumer, D-N.Y., sought MacDonough’s advice with a revised fiscal 2021 budget in mind. But his spokesman’s statement hailing her guidance Monday night said “no decisions have been made” on a procedural strategy and that “some parameters still need to be worked out.”

Here’s what we know about the process:

What is Section 304 and how has it been used?

Section 304 has been used only once — in early 1977, after President Jimmy Carter asked Congress for an economic stimulus package. But reconciliation wasn’t utilized for that or any other bill until 1980, so the use of Section 304 for that purpose hasn’t been tested until now.

The provision says “any time” after a budget resolution for a given fiscal year has been adopted, as long as it’s “before the end of such fiscal year,” lawmakers “may” adopt a concurrent resolution “which revises or reaffirms the concurrent resolution on the budget for such fiscal year most recently agreed to.”

The conference report accompanying the 1974 law provides a little more explanation.

The bill’s managers wrote that circumstances under which Congress might take up an additional budget resolution include “consideration of supplemental appropriations or pursuant to the issuance of updated figures for the current fiscal year in the President’s budget.”

“Furthermore,” the report said, “whenever there are sharp revisions in the revenue or spending estimates or major developments in the economy, it is expected that Congress would review its latest budget resolution and consider possible revisions.”

Why can Section 304 be used for additional reconciliation bills?

In 2001, then-Senate Parliamentarian Robert B. Dove advised that lawmakers could use reconciliation once per budget for each type of instruction allowed under the 1974 law: for spending, revenue and changing the statutory debt limit. In practice, that means there could be as many as three separate reconciliation bills or just one or two if it combines any of those aspects.

Since the intent of reconciliation is to align spending, revenue and debt with targets in the budget resolution, some experts have opined it’s only logical that revised budgets should be able to spawn additional reconciliation bills. MacDonough apparently agrees with that general principle.

Here’s what we still don’t know:

Can a revised fiscal 2021 budget resolution be used for all three types of reconciliation?

That would be the “logic” of the parliamentarian’s advice, “but she has not yet said so in so many words,” a Democratic aide said on condition of anonymity to protect private conversations.

Republicans aren’t so sure. While the law is fairly clear that another fiscal 2021 budget resolution can be adopted before the end of the fiscal year, MacDonough wasn’t explicit that a revised fiscal 2021 budget could trigger a new set of reconciliation instructions.

Even if it could, the type of reconciliation allowed isn’t obvious. One former Senate GOP budget aide argues that a second reconciliation bill this year could be used only to raise the debt limit, because that’s the one prong of the reconciliation process that wasn’t used in the initial fiscal 2021 budget.

Even that may be helpful to Democrats, however, since they are facing a late summer-early fall deadline to raise the federal borrowing limit — or risk a devastating default on U.S. government obligations that would erode investor appetite for Treasury debt and cut off payments to federal benefit programs, contractors and more.

Could lawmakers start work on a fiscal 2022 budget resolution before the second 2021 reconciliation bill passes?

Start work on it, yes. Complete it, maybe not.

The question came up in 2017, when the GOP-controlled Congress tried unsuccessfully to use a fiscal 2017 reconciliation bill to repeal the 2010 health care law, while planning to then use the fiscal 2018 budget process to pass their tax code overhaul.

Many experts believed at the time that the Budget committees could begin work on a new budget resolution before Congress finished the previous fiscal year’s reconciliation bill. But they also thought that as soon as the final budget resolution was adopted, the previous reconciliation instructions would expire.

“We can do everything right up to the finalization of the conference [on a fiscal 2018 budget resolution] without wiping out the right for reconciliation,” then Senate Budget Chairman Michael B. Enzi, R-Wyo., said. MacDonough did not formally opine on that question in 2017, so uncertainty remains.

This matters for the timing of the already delayed fiscal 2022 appropriations process, as well as the next round of reconciliation Democrats want to carry out, whether it’s for Biden’s forthcoming education and child care package or other priorities lawmakers want to attach, like liberalized immigration rules.

Top Democrats say the infrastructure bill may pass this summer at the earliest, which means if they use a fiscal 2021 reconciliation bill, they won’t be able to adopt a final fiscal 2022 budget until potentially September.

It may also be difficult to write a fiscal 2022 budget resolution without knowing what the budgetary impact of a second fiscal 2021 reconciliation bill would be, one budget expert said.

Plus, planning for a new reconciliation bill under the 2022 budget while its predecessor is still under consideration would put an “unusual burden” on the Congressional Budget Office and Joint Committee on Taxation, which are required to estimate the cost of legislation and amendments, he added.

How many times can a budget be revised?

Unclear. MacDonough’s opinion said only a revised budget “may” include reconciliation instructions — nothing about how many times that could occur.

And the statutory language conflicts with the statement of managers in the conference report on the 1974 law.

The text of Section 304 says lawmakers “may adopt a concurrent resolution on the budget” revising the most recent budget for that fiscal year; note the word “a,” which implies the singular. However the conference report says Congress “may adopt at least one additional resolution each year” on top of the earlier budget; note the use of “at least one” in this instance.

Parliamentary debates aren’t the same thing as actual litigation, but one aspect is similar. The plain language of statutory text is the most important determining factor. If the language isn’t clear, participants in the discussion often look to the legislative history and explanatory materials for guidance.

Can a revised budget break new ground, or does it need to be “germane” to the earlier budget?

This is a critical question for the contents of a reconciliation bill, which depend on the instructions to House and Senate committees — typically ceilings on deficit increases or a minimum level of deficit reduction.

Some question whether lawmakers can increase deficits beyond what was laid out in an initial budget, though if the third fiscal 1977 budget resolution is any guide, the answer would be “yes.” That measure made room to add about $19 billion to the deficit expected under the previous resolution for the fiscal year ending Sept. 30, 1977.

Another question is whether a revised budget resolution could issue reconciliation instructions to committees that did not receive them in the earlier budget. But luckily for Democrats, the initial fiscal 2021 budget gave instructions to pretty much every big committee on Capitol Hill.

And even those that didn’t get an instruction, like Senate Energy and Natural Resources — where top swing vote Joe Manchin III, D-W.Va., is chairman — could make their presence felt in House-Senate negotiations because their House counterparts drafted parts of the bill.

Do they really want to do this given the pain of “vote-a-rama”?

A potentially limiting factor is the prospect of multiple exhausting “vote-a-rama” sessions in the Senate to the exclusion of other business.

As a former Democratic aide put it, getting the votes for a budget resolution, going through the Senate vote-a-rama and writing and passing a reconciliation bill even once a year “is painful, arduous and extraordinarily time-consuming under the best of circumstances.”

Peter Cohn contributed to this report.