The :members keyword to defsystem is used to specify the members of a system. The argument given to :members is a list of strings. A system member is either a file or another system, identified by a name. If a full pathname is given then the function pathname-name is used to identify the name of the member. Thus, for example, the name of a member expressed as /u/neald/foo.lisp is foo .
The behavior of any member within a system can be constrained by supplying keyword arguments to the member itself. So, for example, specifying the :source-only keyword ensures that only the source file for that member is ever loaded.