A flagmark is a lightweight per-Subject fixed set of flags. You could meet a different terminology in other engines like “tag components” or “flag components” for a similar functionaly. In Apparatus it is essentially a 32-bit integer that can be trivially tested at runtime.
The flagmarks are always matched at runtime, during the iterating/operating process exactly. They are not cached or pre-matched in any way. This should not be a performance issue though, since the checks are just integer being binary op’d with two other integers and that’s it.