That property only applies if you are using the HSM client.  It means that a
backup version must exist in a TSM backup storage pool before the file is
even eligible to be pushed out to an HSM storage pool due to x days of
non-use.  (The reason for the property has to do with how you would recover
a damaged HSM pool, disater recovery, etc)

Unfortuately there are places where the manuals use the term "migration" for
two different things 1) using the HSM client for disk grooming, which
actually MOVES seldom-used files off the client machine to a TSM-HSM pool,
and  2) relocating backup versions from a primary disk pool to a sequential
(usually tape) pool.  It's confusing.

I think it may be explained in the old old redbook that described how the
HSM client works, if that book still exists...