Thou shalt put the mercy seat above the ark. Rather, "upon the ark"—"thou shalt cover the ark with it." This had not been expressed previously, though the dimensions (Exodus 25:17), compared with those of the ark (Exodus 25:10), would naturally have suggested the idea.
In the ark thou shalt put the testimony. This is a mere repetition of Exodus 25:16, marking the special importance which attached to the provision.