I don't understand what an active s-box is.
(in reference of designcriteria 8 of the DES)

Because e.g. by the DES all 8 s-boxes are used every round.

But this is not was active means.
So my question. What is an active S-Box and what it means when a s-box is active or when two or three s-boxes are active?

Thanks for help.