If you are on the old tab system, then you will accumulate a tab for a future trade up to a maximum value of 150$. If you are on the new tab system, once your tab reaches 0, you just stop paying the tab fee.
I just signed up with Koodo recently and I don't have a tab since I did not get the phone from them. Also, I did not get a 10 percent discount on my plan. But I think the advantage is I can switch to Promo Plans no problem since I don't have a tab. :-)