Equation of state

Outrageous

I know Pv=RT, but I don't understand why my book said f(P,v,T)=0 mean?
Why the function of P,v,T equals to zero? Why not equal to a constant?
Thank you.

Outrageous	Equation of state Tweet I know Pv=RT, but I don't understand why my book said f(P,v,T)=0 mean? Why the function of P,v,T equals to zero? Why not equal to a constant? Thank you.