I am trying to calculate the force a solenoid will develop using the following equation:

F = (N*I)2 μ0 A / (2 g2)

Where:
μ0 = 4π×10-7
F is the force in Newtons
N is the number of turns
I is the current in Amps
A is the area in length units squared
g is the length of the gap between the solenoid and a piece of metal.

My question is what geometry the area (A) and gap (g) represent?

Does Area (A) represent the cross sectional area of the iron core or the coil? Is it the area of the round face or the rectangular cross section?

Does the gap (g) represent the gap between the outside radius of the iron core and the inside radius of the coil or something else?

Thanks