all-before and all-after options

[Glenn A. Carlson, P.E.]

I've seen several instances of the following make statement:

all: all-before xyz all-after

where xyz is some filename usually of an executable or a library to be
created.

The "Complex Make File Example" in the gnu make documentation shows all:
followed by a list of other targets.  This makes sense to me, but I can't
find anything in the gnu make documentation which describes all-before and
all-after, and the makefiles I've seen elsewhere which include all-before
and all-after don't include them as targets.  A Google search on 'makefile
"all-before" "all-after" ' will return several examples of what I'm talking
about.

(A similar question arises for the 'clean: clean-custom' make statement.
What does 'clean-custom' do?)

Can anyone help me understand what this statement and what all-before and
all-after are doing?

Thanks for the information.

Glenn Carlson