Concurrent Manager and APPLPTMP parameter

Digesting the information in this article, it reminds me that sometimes it is the smallest of items which have the largest of impacts.  While the author starts off telling us about the ORA-20100 error encountered for FND_FILE, the important meat here is what the parameter $APPLPTMP is and just how it is different from APPLTMP.  I found this interesting because I'm not quite sure what $APPLPTMP is set to in our environment, and I'd like to investigate this a bit more in 11i as well as when we go to R12.