What does XA stand for in P&ID

Anyone know what XA stands for in a P&ID drawing? The drawing legend does not say

X - user defined
A - alarm

AustralIan is correct. Just want to expand on this a little. X is user defined, but rarely is it actually defined on the drawing. Most of the time X serves as a "Catch All" for everything that is not normal defined in P&ID's. Things like Pressure (P), Temperature (T) and Flow (F) are self evident but a X may be many different things depending on the device. That is why it will not be defined on the drawing.

Thanks

As AustralIan and Bullzi have indicated, the XA means User defined Alarm. This is defined by ISA-5.0-1984(R1992) Instrumentation Symbols & Identification; a standard for the process industry to identify symbols on a P&ID; I would attach the Table 1 from this standard, which contains a complete (?) list of the identification letters and their use, but it is copyrighted. Basically, the standard defines a 2, 3 or 4 letter descriptor with a numeric identifier. This is a widely accepted standard; it can be purchased from ISA - The International Society of Automation at http://isa.org , or downloaded if you are a member.

Note: ISA was the Instrument Society of America way back when I joined