I will take a shot at this, although I am not 100% sure which sub you are referring to. In any key you can generally substitute the chord which is either a third above or a third below the indicated chord (using the notes of the diatonic scale). If you think about how chords are formed and extensions added, you are just moving one position up or down the sequence of notes. If we look at the key of C, and if you are saying can you sub a B minor seven flat 5 (or B half diminished) for the G7 chord, I would say yes. You would end up with the notes B,D,F,A. This would essentially be a G9 chord minus the root.

Hope that helps.
Bob

Bob has it right. You can sub the 7dim chord for the 5dom chord and vice versa. You can always extend a chord by adding the 9th to it. If you then don’t play the root, you are simply playing the chord built from the third of the original. So G7 becomes G9 = g b d f a. Chop off the root, and you are left with b d f a, which is B-7b5. You can also sub the full diminished chord, which has Ab instead of A. In that case the G7b9 becomes Bdim7. Both work because both extensions are fine on a dominant chord.