When enabling group parenting, all layers that have no parent or parents outside of the group will be parented to its group header. Parents within the group, however, are allowed. If a group has sub-groups, all layers, contained in the sub-group, will be parented to the sub-group header. (If you try to unparent one of those sub-groups, they will be parented again.) If layers are moved into a parented group and Refresh Layout is executed, they will be parented as well.