How to understand the N + 1 axis of press brake machine?

3 + 1 axis: Y1,Y2, R and crowning axis
4 + 1 axis: Y1, Y2, X, R, Z1, Z2 and crowning axis
5 + 1 axis: Y1, Y2, X, R, Z1, Z2 and crowning axis
6 + 1 axis: Y1, Y2, X, R, Z1, Z2 and crowning axis
8 + 1 axis: Y1, Y2, X1, X2, R1, R2, Z1, Z2 and crowning axis