- # real UID and not, for example, -1. That is taken care of in
- # src_configure() for now.
+ # real UID and not be e.g. the empty string.
+ #
+ # Our sys-user_create() function makes sure to set SYS_USER_UID to
+ # something useful, and the only place that sys-user_create() is
+ # called from is sys-user_pkg_preinst(), which takes place before
+ # this sys-user_src_install().
+ #
+ # The other way that SYS_USER_UID could be empty is during an
+ # upgrade; however, if you're doing an upgrade and the new UID isn't
+ # important, then you'll get the same old UID that exists on the
+ # system from the old package. That old UID is assigned to SYS_USER_UID
+ # in sys-user_src_configure(), so that case is handled too.