Each routine in the PMDF API has two calling formats: a Pascal-style format and a C-style format. The only difference between the two is the mechanism used to pass string data: the Pascal-style format uses string descriptors, the C-style format uses pointers to strings. All routines return VMS-style status codes - if the low bit is set, the routine was successful. The strings returned by the C-style routines are null-terminated, but strings passed in to those routines need not be.

The API routines fall into three classes: routines to enqueue a message, routines to dequeue a message, and miscellaneous routines which typically query or set PMDF states. The use of the enqueue and dequeue routines is discussed at length in Sections 1.2 and 1.3.

A working knowledge of RFC 822 ² and the relevant sections of RFC 1123 ³ is essential to programmers writing software which will create electronic mail messages with PMDF. Programmers interested in creating MIME-compliant messages should also familiarize themselves with RFCs 2045 and 2046. ⁴

Note that channel programs written using the API should always use the PMDFlog routine to write output to the channel log file.