The difference between the two different types of Geometric Set are the order of the elements within the set.
In the normal GS, the sequence of elements doesn't matter. You can re-order things anyway you want (by element type, alphabetically, etc) with no problems.
In the OGS, the sequence of elements is important: Parent geometry must come before (be higher in the tree) it's childred geometry. If things are out of parent/child order, you typically get an Update Cycle error. You must be very careful when elements are re-ordered in a OGS.