A frozen ATPL is a term used mainly by recruiters to define having a cpl with an spa IR and ATPL exams passed. Arguably completed an MCC course too. From licencing point of view, there is no such thing as a frozen ATPL, which is why you won't find a reference.
Once you have this, the cpl is valid for 5 years and the IR is valid for 12 months. If you let the IR expire by more than 7 years, you must retake all 14 exams to get the IR back. Once you get an ATPL you should not have to do the exams again, and will then have an mpa IR anyway.