Merkle proof cells verify that some cell tree data belongs to the full tree. See Merkel proof cells page for details.
This design allows the verifier to avoid storing the entire tree’s content while still being able to verify the content using the root hash.A Merkle proof cell contains exactly one reference c. Its level 0 <= l < 3 is max{Lvl(c) - 1, 0}.Each Merkle proof cell serializes as follows: