What is exactly the role of @tsmbrieflist argument?
TSM allows for a small buffer of header information to be passed when an object is created. That information is stored in TSM catalogs somewhere. Those catalogs are always online. The data or actual backup however is stored per the storage pool which can be on tape or disk. The size of the max length allowed in that header buffer only allows you to store so much information in it. @tsmbrieflist=1 (default) only retrieves information which is stored in the header. It can therefore be more faster compared to using @tsmbrieflist=0. Using @tsmbrieflist=0 accesses the actual xml which could be stored on tape and can take hours to gain access to.