When you're moving forward, you are the person taking the step outside of your partner's track. When your partner is moving forward, they are taking the step outside of your track. So whoever is moving forward is said to be stepping "ouside partner", while whoever is moving back is said to be stepping with "partner outside".

Incidentally, the "partner outside" terminology is really our own. Most technique books will only indicate "outide partner" for the person moving forward, and will not specify anything at all for the person moving back. But we think it's an important piece of information to know, especially for the man if he's supposed to be leading the lady to step outside of him. So we include it as "partner outside" as a bonus at no extra charge.

Sincerely,
Jonathan Atkinson
www.ballroomdancers.com