Is it on at all

Enabled, per collection. Off means this collection is skipped entirely — it never builds, never gains or loses titles, and Build-all passes it by. Its Plex collection is left exactly as it is rather than removed, so switching a collection off is not a way to delete it.

Report unreadable sources. Off means an unreadable source on this collection is expected and never reported. Use it where a builder points at something that may simply not be linked — somebody's watchlist, a tag nobody uses. A build that fails outright is still reported either way.