PSV calculation as per API 520,
F=W /366 * (kT/(k+1)M)^1/2 + AP

One calculation sheet i have defines calculation method as followings;
W /366 x (kT/(k+1)M)^1/2 = F1 applied for Horizontal Dire.
A x P = F2 applied for Vertical Dire.
(where P = 0.00245 W/(d2)^2 x (T/kM )^1/2 )

and another sheet defines for F2 :
F2 = ( w^2 x R x T ) / ( g x Ps x A )
F1 is same.

For DLF, F1 is only applied with 2.

My question is.
1. What is difference of F2 mentioned in above.
2. Why does it apply DLF only for F1.