Files with xap suffix might also be potentially found as old scripts for Xbox 360 video console. Not related to later Silverlight or XACT packages.
There are 2 other file types using
the XAP file extension!
.xap - Microsoft Windows Phone and Microsoft Silverlight application package [common]
.xap - XACT project file
XAP file extension - Xbox dashboard script file
What is xap file? How to open xap files?
File type specification:
The xap file extension seems to also be the Xbox 360 video gaming console from Microsoft and used for some kind of script that was part of the original operating system.
Unfortunately, we couldn't find more information and it may be possible that this file type is no longer used. But it looks like that this file type is not related to later Silverlight or XACT packages.