Multiple blog types vs. Multiple Subtypes

Please explain the mutual exclusiveness of allowing Multiple blog types and Multiple Subtypes structurally. Is this universally determined by super-admin with the allow statements on the blog-types-config.php It appears you can enable subtypes and allow for both. However, the note that says "if you allow users to select multiple blog types, they cannot select a subtype" is not clear to me. If you allow for both, do users have a choice to either have multiple blog types or multiple sub-types? Or should we only allow for one alternative?