This is correct.  The pattern of volume usage for collaction is copied and
pasted below (from the 3.7 Admin Guide, page 156-157).  You're right about
predefined volumes and low or zero maxscratch.

Alex Paschal
Storage Administrator
Freightliner, LLC
(503) 745-6850 phone/vmail

How the Server Selects Volumes with Collocation Enabled
When collocation at the client node level is enabled for a storage pool
(COLLOCATION=YES) and a client node backs up, archives, or migrates files to
the
storage pool, the server attempts to select a volume using the following
selection order:
1. A volume that already contains files from the same client node
2. An empty predefined volume
3. An empty scratch volume
4. A volume with the most available free space among volumes that already
contain
data

When the server needs to continue to store data on a second volume, it uses
the
following selection order to acquire additional space:
1. An empty predefined volume
2. An empty scratch volume
3. A volume with the most available free space among volumes that already
contain
data
4. Any available volume in the storage pool