When you see the symbols used for identification of piping class designations you need to know the meaning of them.

Piping class designations consists of a maximum of four symbols normally.

The first indicates flange rating, the second indicates corrosion allowance, the third indicates materials of construction, and the fourth denotes the service.

The flange ratings and facings may be Raised Face Flanges,Ring Type Joint Flanges, Lap Joint Flanges,Flat Face Flanges,Non-flange rated systems with rating class150, 300, 600, 900, 1500 or 2500. They may be indicate as A, B, C, D, E, F,…..

Corrosion allowance may be 0.00mm, 1.5mm, 3.0mm, 4.5mm, 5.0mm, 6.0mm.They may be indicate as 0,1,2,3,4,5…

For examplae piping class A1AA
where 
A ( first symbol) is indicated for Raised Face Flanges - Class 150
1 ( second symbol) is indicated for corrosion allowance 1.5mm
A ( third symbol) is indicated for piping marterial ( like Killed Carbon Steel)
A ( fourth symbol) is indicated for service ( like Instrument Air )