Fernco with "separator" and without, what is it for?

[Mikieboyblue]

I have been curious about something, why do some Ferncos like the ProFlex line (say 3005-33) have interior ridges and a "separator" and others are smooth (say 1056-33 RC)?

 

[Mikieboyblue]

Is it just meant to be a stop vs slip?

 

[House Doc]

Yes.
Where you can't pull the pipes apart more than 2" you use the "slip" type.