Baltic Birch is not a specific species of Birch, rather, a general designation of plywood from Russia, Finland, and nearby Baltic states. Baltic Birch plywood is generally regarded as being of higher quality than standard Birch plywood.
Baltic Birch plywood is made entirely from Birch plies, with no softwood or filler plies. The plies themselves tend to be much thinner, allowing for more plies for a given thickness, and greater stiffness and stability.