ADSM will never allocate more than 'MAXSCRatch' Volumes for the StoragePool: Collocation becomes defeated when the Scratch pool is exhausted as Adsm will then mingle Clients. When a new Client's data is to be moved to the storage pool, Adsm will first try to Select a ScratchTape, but if the storage pool already has "MAXSCRatch" Volumes then it will Select a Volume as follows: For Collocation by Node: Select the Tape with the lowest utilization in the storage pool. For CollocationByFilespace: first try to fall back to the collocation-by-node scheme by selecting a Volume containing data from the same ClientNode; then Select the Tape with the lowest utilization in the storage pool.
Ref;
Admin Guide, chapter on Managing StoragePools and Volumes, topic How the Server Selects Volumes with Collocation Enabled
See also,
MAXSCRatch