confusion about property definitions

["Walter Purvis" <wpurvis@datamobilitygroup.com>]

Forgive me for being so dense, but I'm having trouble wrapping my head
around something.

In the Property Definitions for Documents section (2.1.4.3.3), there are a
dozen or so properties (e.g., objectID, createdBy, etc.) where the property
attribute Required = False, but there's a not that says "MUST be set on the
object." (The same is true in Property Definitions for Folders,
Relationships, and Policies).

Can someone enlighten me as to the meaning of Required in this context, and
also the meaning of the "MUST be set" part?

Thanks.