I've been a member of Stack Overflow for years and I also read other sites like https://security.stackexchange.com. This morning while I was logged into https://stackoverflow.com, I then went to security.StackExchange.com. There, it said I was not logged in and prompted me to create an account. I then tried logging in with my Stack Overflow data but it said not valid or something to that effect. Apparently these sites are linked but do not seem to honor each other's login. So I posted a question as a guest.
Later I desired to post a comment or even vote up the excellent answer I received to my guest question. But I could not. So in frustration I clicked create new account and gave it my exact same email, username and password that I use on Stack Overflow. It said nothing about any existing account but rather welcomed me to the new account.
So I presume even though these sites are all linked they still have separate logins and the reputation points etc do not apply to other linked sites. Is that correct?