When you have done either another class or type test. eg MEP or 747. TRE's can't sign your SEP's off even though someone that has filled the paperwork out and payed the dosh and become a ground examinor with no training at all can.
You can go years and years without ever having flown with an SEP instructor if you fly other things.
Which is one of the many reasons why I think that the 1 hour with an instructor is a load of pish.
And in case it comes up yes there are tre's out there who have all the ticks in the boxes and can sign you but if they are "just a TRE" on one type with no other auths they can't.