Under what circumstances might the FCC issue a Special Temporary Authority (STA) to an amateur station?
Why A Special Temporary Authority is granted under Part 1 (Section 1.931) and lets a station operate in a way the normal rules do not allow, for a limited time, usually up to 180 days. For amateurs the FCC has used STAs to permit experimental communications, such as testing on a frequency, mode, or power level outside normal Part 97 limits. It is an exception to the rules, not a routine administrative convenience.
Watch out Special event 1x1 call signs are the tempting pick, but those are coordinated through the volunteer special event call sign system, not by an FCC STA; likewise, a newly upgraded licensee simply self-identifies with the /AG or /AE indicator and needs no special authority, and the three-VE minimum has no exception.
STA = experiment beyond the rules for a short time. Routine things (1x1 calls, /AE upgrades) never need an STA.