Frame Type V vs X

Hello, perhaps somebody can explain once, when one has to use Frame Type V instead of X.
My understanding so far is, that you use V when the front arms are more angled then the back arms (same armlength) on a quadcopter (i.e. TBS discovery pro). But I have seen now that the default frame type for a 3DR IRIS+ is V although the front angle is the same as the back angle.

Thanks for clarification. It would be also good to include that in the documentation