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.