True if the target is multiple objects

Does the target object still exist?

A list of actual target objects - if applicable

It's good manners for a changed SerializedProperty to tell its parent on set. That way the parent can cascade changes up the tree. This is particularly important if the tree includes struct types - because those values will need to be re-set on any ParentProperty's.

Called right before enumeration, to allow derivitives react to changes