+@defvr {Scheme Variable} channel-build-system
+This variable is exported by @code{(guix build-system channel)}.
+
+This build system is meant primarily for internal use. A package using
+this build system must have a channel specification as its @code{source}
+field (@pxref{Channels}); alternatively, its source can be a directory
+name, in which case an additional @code{#:commit} argument must be
+supplied to specify the commit being built (a hexadecimal string).
+
+The resulting package is a Guix instance of the given channel, similar
+to how @command{guix time-machine} would build it.
+@end defvr
+