This article is from the Dissociation FAQ, by Discord (email@example.com) with numerous contributions by others.
This (like everything) varies from system to system. In many systems,
the alter with the veto power is the core, the host, or both. Sometimes
guardian alters have the most control.
Sometimes everyone has control; anyone can wrest control away from anyone
else. This, as you can imagine, has its downside, but it works fairly
well for a number of systems.
In a well-integrated system (co-conscious), things are often decided by
a set number of alters, through a strictly democratic system, or
something similar to that. There may or may not be alters with effective
veto power in these cases.