close-reasons is for questions regarding the site-specific pre-defined close reasons you can find under Close → Off-Topic.
on-topic is about what we consider on-topic on this site and what not.
As the pre-defined close reasons are only for common types of closure, not every discussion on on-topic-ness pertains to the close reasons. On the other hand, e.g., fixing a spelling mistake would only pertain to the close reasons, but not to what is on-topic or not.
I added a description explaining this to close-reasons.