HTTP 402
The HTTP status code reserved in 1997 for "Payment Required" and left unused for twenty-five years.
a protocol · the vocabulary
402Payment Required
Every other 4xx code was specified and adopted; 402 sat in the standard marked "reserved for future use" because nobody agreed on how a server should ask a client for money. It is now being revived as the basis for machine-to-machine payment, where the client is an agent with a wallet rather than a person with a card.