Also known as a division board it's the full internal dimensions of the hive, with no bee spaces around it, it can be used to 'partition off' part of a bee hive, to render it less useful to the bees or it can be used to fill minor gaps at the ends of groups of frames.
In some circumstances it can be used to separate independent nucs.
Plywood construction, equivalent of 2nd quality hives - some wood knots may be present.