Baltic birch plywood is unlike any other. It is only produced in regions of northeastern Europe, in the countries surrounding the Baltic Sea. It is produced primarily for European cabinet making and can be tricky to find here in the states.
The source of the plywood's strength is in its construction. Thirteen plies of 1 mm-thick solid birch veneer is cross-banded and laminated with a strong adhesive. This method produces a void-free core that makes for very solid joints.
Baltic birch plywood will resist warping and moisture far better than domestically-produced plywood. This means that if you put your cornhole boards onto a wet lawn, the water won't seep into the wood, causing the frame to break down.