I was always of the understanding that a "Frozen" ATPL means one has the SAMRA / SARON written then they get a type rating on a 2 crew required aircraft. On successful completion of the PPC the requirement to obtain the ATPL within 2 years is waived and the exams are "frozen" as completed, indefinately. The same rule applies to the IATRA.
This only applies when obtaining a PPC on an aircraft that requires 2 crew on its type certificate with the exams written.
Cheers
DW