It is a two-lines change that allows a defsystem form depend on a package that provides additional functionality, such as new system classes, new system options, new methods, etc.
:ASDF-DEPENDENCIES ({system-name}*)
fee.asd:
(defsystem :fee
:components ((:file "fee")))
foo.asd:
(defsystem :foo
:tests ((:system :faa))
:asdf-dependencies (:fee)
:components ((:file "foo")))
> (load "asdf")
;;; Loading #P"/Users/jjgarcia/devel/asdf/asdf.fas"
;;; Loading #P"/Users/jjgarcia/lib/ecl-10.3.1/CMP.fas"
;;; Loading #P"/Users/jjgarcia/lib/ecl-10.3.1/sysfun.lsp"
#P"/Users/jjgarcia/devel/asdf/asdf.fas"
> (asdf:load-system :foo)
; loading system definition from /Users/jjgarcia/devel/asdf/foo.asd into
; #<ASDF0 package>
;;; Loading "/Users/jjgarcia/devel/asdf/foo.asd"
; loading system definition from /Users/jjgarcia/devel/asdf/fee.asd into
; #<ASDF1 package>
;;; Loading "/Users/jjgarcia/devel/asdf/fee.asd"
; registering #<SYSTEM :FEE 11882808> as FEE
;;; Loading "/Users/jjgarcia/.cache/common-lisp/ecl-10.3.1-darwin-x86/Users/jjgarcia/devel/asdf/fee.fas"
;;;
;;; Loaded prerequisite FEE
;;;
; registering #<SYSTEM :FOO 12258736> as FOO
;;; Loading "/Users/jjgarcia/.cache/common-lisp/ecl-10.3.1-darwin-x86/Users/jjgarcia/devel/asdf/foo.fas"
#<ASDF:LOAD-OP NIL 9826760>
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