The Ulster Unionist Council is the governing body of the UUP and comprises some 900 members drawn from a number of levels within the party.
Representatives from each of the Unionist Associations in the 18 Westminster constituencies are entitled to attend and vote. The bulk of those attending today's Ulster Hall meeting are drawn from this tier of the party.
The Orange Order is also formally represented, with about 120 or so attending on the basis of the formal link between the loyal order and the UUP.
The women's section of the party also sends a delegation as does the young Ulster Unionists.
The remainder of the attendance is drawn from the elected representatives. These include those elected to the North's 26 local authorities, the Assembly, the House of Commons and the House of Lords, and the European Parliament.
The UUC is summoned when a requisition signed by 60 of its members is delivered to Ulster Unionist headquarters. There have been 12 such meetings since Good Friday, 1998.