Fill/Kill is a convenience only. Let's say you are going to be away from your computer for the rest of this week and you expect some big news on stock WXYZ. Let's say the price is $1.00, but you think it might spike up to $1.60. If it does, you want to freeze it. But, you aren't at a computer. You can submit your Fill/Kill request to this board (basically SSK) and if it 1) hits $1.60 within a week or before you cancel the order, he will fill your request. At the end of a week, the request is killed - if you haven't killed it before then.
Simply put, it is a 1 week "fill or be killed" request for a specific transaction. You can only have one outstanding at a time, but you can renew the request each week if you want. That part of it is subject to change. I think it could become a problem, but SSK isn't worried about it. So, we will play it by ear.
I should say that Fill/Kill applies to any type of transaction. Maybe you want to unfreeze a stock if it drops to X price. Or, maybe you want to trade ABCD for EFGH if ABCD hits $2.10 and EFGH is below $4.00 on the same close. I wouldn't try to get too fancy with it, but theoretically, any of our basic transactions could be applicable to Fill/Kill.