From The Free On-line Dictionary of Computing (19 January 2023):
Transaction Processing Facility
TPF
<operating system> (TPF) A real-time mainframe operating
system released by IBM around 1976. TPF is particularly
suited to organisations dealing in very high I/O message
switching and large global networks. Current users include
British Airways (reservations), VISA International
(authorisations), Holiday Inn, and Quantas.
TPF was traditionally a 370/Assembler environment although
the latest, release 4.1, contains C. Formerly known as ACP
(Airline Control Program), it was renamed "TPF" to suggests
its greater scope.
It is common for TPF sites to use IBM's MVS and VM
operating systems for off-line processing.
(1996-08-27)