Binders determine where a vfx should be created, and its tracking behavior (should it follow a moving character).
There are several types of binders:
Point (the most common, attach to a character, for example)
Binders can be attached to caster or target, and have several "property" fields. Properties have the attribute
BinderPointId, which allows a vfx to be bound to a specific point on a model (such as at the hilt of a sword). Binder points are specified in the skeleton's
.mdl file, although this is not well-understood.