Mechanical Design Pressure / Temperature

From time to time I've come acrros these terms Mechanical Design Pressure and Mechanical Design emperature. Have any of you a writen concept of these terms.
I haven't been able to find them in any standard, and averybody that I ask has a different version/ definition... The worst part is that I've seen stress calculations based in these terms and nobody dares to argue about what/where are these numbers coming from.
Can you give some insight?

Thanks

I've seen these terms used on Offshore jobs where it refers to the Design Pressure and Design Temperature to be used for mechanical strength (wall thickness and component selection). These numbers generally match the piping specification limits. Where this is the case, a flexibility temperature is usually given that reflects the system's real temperature to be used for pipe stress analysis considerations.

In short:
MDT = DT = T Limit of Spec
MDP = DP = P Limit of Spec
TFLEX = REAL (Operating) TEMP.
With,
MDT as Mech Design Temp
MDP as Mech Desing Pressure
TFLEX as Temperature for Stress analysis.

That is one, any more?
TX