3,628. Things From Which Terumah Need Not Be Taken, But Challah Must

Hilchos Bikkurim 6:3

While one need not take terumah from leket, shich’cha, peah, hefker (ownerless produce) and produce that has not reached a third of its growth, one must take challah from these things. Likewise, one must take challah from dough made from grain whose tithes were taken while it was still stalks and terumas maaser was taken from it. This is true even though the terumah gedolah is still in it. Challah must also be taken from second tithe, consecrated property that was redeemed, the surplus barley of the omer, the two loaves offered on Shavuos, and showbread that was redeemed.

Hilchos Bikkurim 6:4

If dough was made from second tithe in Jerusalem, or from Sabbatical-year grain, or if there’s a doubt that the dough might have been made from meduma (a terumah/secular grain mixture), one must take challah. One need not take challah from dough that is definitely meduma.