This read-write enum is the row-control object for a destination MAC filter entry: a GET always returns valid(1), and setting invalid(2) is how an admin deletes that filter entry entirely. This follows the same simple delete-by-invalidate pattern used elsewhere in this MIB, just without a separate create value, new rows here are presumably created by setting the other columns of a not-yet-existing index. An admin retiring an old MAC filter rule that's no longer needed would set this object to invalid(2) to remove it.