Once upon a time there were ACLs (Access Control Lists) in Documentum. All the internal artifacts used the term. Then they were renamed to permission sets, though internal references continued to use ACL as expected. So both the terms were prevalent.
In version 6.5 the Content Server Fundamentals guide doesn’t even mention the term permission set. It seems to be all ACL again. But wait, Composer uses permission sets liberally.
Does it really matter? Probably not. Then why mess with the terminology repeatedly? I am sure that there are better ways to improve the platform.