"Third party" means that the person trading with you wants you to send something to one account and receive something from a different account. Sometimes that can be legit - many of us manage accounts / credit card signups for family, friends, etc. (and since there's no direct trade it's potentially more discrete vis-a-vis Hyatt)

But it could also mean that you are receiving something from a hacked account, and are therefore opening yourself up to a world of pain. As a result, you should avoid unless it's a particularly active and trustworthy FTer.