Can anyone teach me what is D, E & A stand for in the below attached and how and where we will get these values.

 

consult the vendors data to answer your question

 

The block appears to be a calculation module for steam flow rate through a valve. Steam flow rate is proportional to square root of differential pressure DP in the figure shown as input from another module. D,E,A are the fixed values used in the formula for flow rate.
Without knowing the context it is difficult to answer. If more details are given we can answer.

 

Guess: D = 32.5 Temperature; E and A: density and valve flow coefficient;