Из спецификации XAP-PT :
This document is a CAE Specification (see above). It defines the X/Open ACSE/Presentation
(XAP) programming interface Transaction Processing extension (XAP-TP).
X/Open has already defined an ACSE/Presentation (XAP) programming interface (see
Referenced Documents), which provides for access to the ISO OSI protocol stack at the upper
two layers (Association Control Service Element and Presentation) of the OSI 7-layer model.
Мне кажется, что XA означает X / ACSE