A classifier is a special type of projection that is used to decide if an aggregate is "in" or "out" of a given identifier group.
This allows for performing rudimentary set-based operations over an event sourcing system without the need to worry about locking the read model or running off stale eventually consistent data.
A classifier can either be deinfined with an event filter in the same way as a projection is defined, or it can use and existing projection and perform a logical operation on the return from that projection to decide if an aggregate is inside or outside of the identifier group.