Each Common Lisp implementation has its own format for compiled files or fasls.1 If you use multiple implementations (or multiple versions of the same implementation), you'll soon find your source directories littered with various fasls, dfsls, cfsls and so on. Worse yet, multiple implementations use the same file extension and some implementations maintain the same file extension while changing formats from version to version (or platform to platform). This can lead to many errors and much confusion as you switch from one implementation to the next.
Since ASDF 2, ASDF includes the
to mitigate the problem.
 “FASL” is short for “FASt Loading.”