On the upper right side of the "Child Groups"module there is a green button that says "Add Group". When you click on it you get to name the group, choose the color for the group and select if it is going to be a main or optional group.

The difference between the principal group and the optional one is that a child cannot be a member of an optional group, without being in a principal group already. The optional classes selected by the parents are part of optional groups.

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