Discussions about Vygotsky and the Cultural Historical Activity Theory are popular topics of discussion. For those interested in a further study of Vygotsky this list serve is highly recommended. The task of Vygotsky in 1930-1931 was to create the psychology in terms of drama. The stage is the place the dramatic development takes place. The planes only make sense relative to the stage and they are connected as two projections of the stage where the child is not a spectator, but participant.
Category is the philosophical concept.
Vygotsky is speaking about development as a process of events, collisions and their reflections in both planes.