Deactivate / Delete a Source

Summary: 

Hi :) We want to clean up our 'source' list, however there isn't any option to 'deactivate' a source, so that it can be reactivated at a later date if needed.  The only information I can find is "If the deleted source was being used, it will only be deactivated. If the deleted source was not being used, then it is permanently deleted".

Found here: https://docs.oracle.com/en/cloud/saas/taleo-enterprise/23b/otrcg/t-deletingasource.html#id095190H067Q

However, I do not understand what 'being used' means/is referring to?  

I tested in Staging, and one source actually deleted completely and another deactivated when selecting 'delete' (which is the only 'available action'). However I don't understand why.