Solving the mystery of PRISM - The Week: There are many types of nicknames and special words that the NSA uses.
Some refer to collection tools. Some refer to data processing tools.
Each data processing tool, collection platform, mission and source for raw intelligence is given a specific numeric signals activity/address designator, or a SIGAD. The NSA listening post at Osan in Korea has the SIGAD USA-31. Clark Air Force Base is USA-57.
PRISM is US-984XN.
Each SIGAD is basically a collection site, physical or virtual; the SIGAD alphanumerics are used to indicate the source of intelligence FOR a particular report.