Remember, IBM stopped caring what you put in UNIT many releases ago. It
put the data wherever it darned well wanted to. It just left the
parameter there for compatibility reasons. UNIT was deprecated.
With SSD UNIT has risen from the ashes. The ONLY use for unit to to
specify ssd or not. If you have unit 100 in there and you mimix the file
over and you don't have 100 disk drives you will not have a problem. IBM
treats a value of 100 as (not 255) aka (not ssd).
Does that answer your question?
Nowadays, if you want to force files to reside on certain disks (outside
of the SSD discussion) you have to build an ASP containing those disks.