Problem Adding Characteristics to an Info structure

I'm trying to add new characteristics to an infostructure in R/3, with transaction mc22.

This infostructure is an infosource for BIW.

Actually it has 9 characteristics and I want to add two new.

After I choose the two new ones and click the button "copy" the system give me this error:

Choose no more than 9 Characteristics

What it means? That the info structure only allow 9 characteristics? no more?