The directory of a library is a series of records (entries) at the beginning of the data set. There is at least one directory entry for each member. Each entry contains a member name, the relative address of the member within the library, and a variable amount of user data.
User data is information inserted by the program that created the member. An entry that refers to a member (load module) written by the linkage editor includes user data in a standard format, described in the systems manuals.
If you use a PL/I program to create a member, the operating system creates the directory entry for you and you cannot write any user data. However, you can use assembler language macro instructions to create a member and write your own user data. The method for using macro instructions to do this is described in the data management manuals.