So, what exactly is a caucus and what does it mean for senators who were formerly part of the Liberal caucus?
1. Party caucuses
A caucus can be any grouping of similar parliamentarians, including a women's caucus, a regional caucus or a party caucus. All three major parties meet separately every Wednesday morning, with the Conservatives and Liberals — at least until Wednesday — gathering their MPs and senators together to discuss policy and strategy. The NDP has no senators, so its MPs make up the entire caucus.
To be a recognized party in the Senate, a caucus must have five members. About 20 former Liberal senators met with Trudeau Wednesday morning and decided they would continue to sit together, as the Senate Liberals. They kept James Cowan as their leader and Joan Fraser as their deputy leader, and Cowan said the caucus will maintain the ability to discipline its members — essentially keeping the same structure they'd had before they met with Trudeau.
2. Budget and research
The Liberal Party didn't directly fund its Senate offices, although the House and Senate shared research and policy. The caucus chair directs a research bureau, whose information will also be off-limits to senators from now on.
That said, there's nothing stopping senators and MPs from exchanging ideas and information on issues, something that's already done across parties when MPs decide to work together on a problem.
Senate Speaker Noël Kinsella agreed that Senate Liberals qualify as the second-largest party in the upper chamber, and therefore as official Opposition in the Senate. That means they can keep their budget for the Senate opposition leader, deputy leader and whip's offices, which pays for research and staff.
3. Committees and procedure
Kinsella's ruling was also important for Senate Liberals so that they have standing to negotiate the timing of votes with the government and for some of the speaking privileges granted to leaders. It means their party's whip is recognized, and gives them the ability to put senators on committees. Senators who aren't actually members of a committee can ask questions if given permission by the chair, but cannot vote on committee motions.