A PDS consists of members (internally identical to sequential data sets), registered in a list called the directory.
The BPAM API is similar to basic sequential access method (BSAM), but it adds functionality to process directories.
As a basic access method BPAM reads and writes member data in blocks and the I/O operation proceeds asynchronously and must be tested for completion using the CHECK macro.
FIND positions to a single member, specified by name, which requires a directory lookup on disk, or by address previously retrieved by BLDL.
[3] The operating system requires all executable programs to be stored in libraries because the member's directory entry contains additional attribute information specific to load modules.