David -

The Admin Ref manual description of the Physical and Logical Space
report element from Query OCCupancy is superficial, leading the
customer to believe that the Logical value is just the amount of space
occupied by client files.  According to info I've seen, logical space
also includes headers housing control information for each file, such
as magic numbers, CRC info, segment numbering, server ID, etc.  The
size of the header info can vary as TSM evolves, and so may differ
across TSM releases.  The header size can be seen in the output of the
SHow INVObject command for a given ObjectID.

   Richard Sims   http://people.bu.edu/rbs