Difference between lib*-routines and lib*-sysdep_routines

[Florian Weimer]

extra-lib.mk says this:

# This file is included several times in a row, once
# for each element of $(extra-libs).  $(extra-libs-left)
# is initialized first to $(extra-libs) so that with each
# inclusion, we advance $(lib) to the next library name (e.g. libfoo).
# The variable $($(lib)-routines) defines the list of modules
# to be included in that library.  A sysdep Makefile can add to
# $(lib)-sysdep_routines to include additional modules.

This suggests that sysdeps Makefiles must use lib*-sysdep_routines 
instead of lib*-routines.  But we use both sets of variables in sysdeps 
Makefiles, and as far as I can tell, they are equivalent.

What is the difference, if any?

Thanks,
Florian