What is Xsa?
The term XSA was first used by a security expert by the name of DeadlyData from a site which goes by
XSA was thought to be short for Cross Server Attack, just like XSS in a way, but isn't exactly the same.
However XSA is where one would gain un-authorized access to a website which was on another part of the server and in which you gained access via another means of way (in other words from another location on the server other than the targeted website).
XSA is used to gain access to a website from a different location of a server (i.e another website on the same server)