If you take Eq 1B:

Sa = f( 1.25Sc + 1.25Sh - Sl )

and rearrange it, you have this:

Sa = f( 1.25Sc + 0.25Sh + [Sh - Sl] )

The first part of this equation (in red) matches Eq 1A. The second part of this equation (in blue) can be considered as the "unused allowable left over from the Primary (sustained) case".

In CAESAR II, we refer to Eq 1B as the "liberal allowable", since this equation will give you a higher allowable stress than Eq 1A. To put numbers to these equations, assume both Sc and Sh are 20000, then Eq 1B can range from:

Sa_max = 1.25*20000 + 1.25*20000 = 50000 (when Sl approaches zero)

to

Sa_min = 1.25*20000 + 0.25*20000 = 30000 (when Sl approaches Sh)

Note that Sa_min is Eq 1A.