Tape mounts within IBM tape libraries are handled by SMS internally and the mount messages are suppressed. CA TLMS requires that the mount messages be issued so that it can intercept them and add the subpool name. Without the messages, CA TLMS cannot not support tape subpooling for IBM tape libraries like the VTS.
Oracle (formerly STK) tape libraries and other non-IBM tape libraries do not suppress the mount messages. CA TLMS can intercept these and insert a subpool name, so tape subpooling is supported.