Used of the cover of the ark of the covenant in the Holy of Holies, which was sprinkled with the blood of the expiatory victim on the annual day of atonement (this rite signifying that the life of the people, the loss of which they had merited by their sins, was offered to God in the blood as the life of the victim, and that God by this ceremony was appeased and their sins expiated); hence the lid of expiation, the propitiatory. (BLB.ORG Outline of Bible Usages)

Recently I wrote that there was a foreshadowing image of the New Covenant during the wilderness journey identified in Exodus. This is in reference to the use of the Ark of the Covenant.

Propitiatory is an adjective and as we have learned adjectives typically serve as a modifier of a noun to denote a quality of the thing named, to indicate its quantity or extent, or to specify a thing as distinct from something else. In this case that distinction is the Mercy Seat and nothing else about the Ark of the Covenant.

Leviticus 16:12-15 English Standard Version (ESV)

¹² And he shall take a censer full of coals of fire from the altar before the Lord, and two handfuls of sweet incense beaten small, and he shall bring it inside the veil ¹³ and put the incense on the fire before the Lord, that the cloud of the incense may cover the mercy seat that is over the testimony, so that he does not die. ¹⁴ And he shall take some of the blood of the bull and sprinkle it with his finger on the front of the mercy seat on the east side, and in front of the mercy seat he shall sprinkle some of the blood with his finger seven times.

¹⁵ “Then he shall kill the goat of the sin offering that is for the people and bring its blood inside the veil and do with its blood as he did with the blood of the bull, sprinkling it over the mercy seat and in front of the mercy seat.