The RCng system was developed by NetBSD and FreeBSD have adopted it. _rc_subr_loaded is a flag that both system adopts to speed up the startup.

The FreeBSD approach is to determine whether to run for the whole rc.subr script, while NetBSD approach is to define it as a “:” so rc_subr won’t be loaded.

Which is better?

There is no easy answer. NetBSD’s approach is somewhat more efficient as rc.subr is not loaded at all, and FreeBSD’s approach is more easy for a developer writing their RCng script.

I personally prefer the FreeBSD approach.