# What plane is Area in, in this equation?

I have the equation $$F = (NI)^2\mu_0\frac{\text{Area}}{2g^2}$$ Which calculates the force of an electromagnet. I was wondering if anyone knew what plane Area is taken from? I have heard it is the cross sectional area of electromagnet(which doesn't make sense to me because it already uses the number of turns, N and the current, I), I have heard is is the cross-sectional area of plane b, and I have heard it as the cross sectional area of plane a. I have also heard it as plain "Area".

A ferromagnetic cylinder being acted on by an electromagnet.(Assume this cylinder is being drawn into a solenoid circle-side first)

Personally, I think it uses the cross sectional area of plane b.

Being a physics problem, can anyone confirm whether or not I should be using the area from plane b?