This is one of those acronyms you come across infrequently. STPC stands for “Stopover Paid By Carrier”. I have used this facility only twice and have flown over 3 million miles. You are not using STPC when an airline just put you to a hotel due to delay, misconnect, cancellation etc. Those are just regular accommodations during irregular operations.
The irony is that these “stopovers”, less than 24 hours, are not actual stopovers but rather transfers. If the transfer is more than 24 hours, it is then considered to be a stopover that might affect the ticket price and the airport taxes.
How to know if you are eligible for STPC? This is a tricky part. It is borderline impossible if you are using one of the online booking sites. Only competent travel agent can probably tell you when this service is available, and if there is a fee for it. The STPC is not usually listed on the fare rules either. Normally, the required connection must be at least 6 to 8 hours when there are no other flight options available between the origin and destination. It is not always offered for economy passengers when it is for business or first class ones, or there may be a charge for economy passengers and free for business first. I have seen this only to apply for revenue tickets i.e. award tickets in business or first are not usually eligible.
Depending of the airline you may need to request this service at the time of ticketing, few days before departure through the airlines office, or sometime upon arrival to the connecting airport.
My two STPG (Stopover Paid By Carrier) Examples:
Czech Airlines Business Class Damascus to Helsinki via Prague