Unfortunately I can not find it now. I think it was in the guidelines for cache placement when I read them about 3 years ago.
The purpose, as I remember, was to separate correspondence for cache placement from other correspondence.
I have checked a couple of reviewers who have published local caches (Long Man & Castagnari) and they have 38 & 44 finds respectively. I imagine that most of their geocaching is done using other IDs.