Using analytical expression

Dear all
I have a question about analytic function, as I see for example as a default for air :
It is used "rho" for "density " of material
Then it is used in analytic function : "argument: pA" and expression : pA*0.02897/8.314/T
After that in the "derivative" it is use manual and mention:
"argument :pA" and "partial drivative:d(pA*0.02897/8.314/T,pA)" I would like to ask what is the meaning of this and how it is working? it means
derivative of (pA*0. 02897/8.314/T, pa) is used in an expression ( pA*0.02897/8.314/T)?

Thanks in advance