Is my understanding of semi-custom cabinets, correct? What common mistakes should I avoid if I want to DIY it?

From what I understand, semi-custom cabinets are stock mass produced cabinets that come in standard sizes combined with custom face frame, side panels, doors and hardware?
Do the face frame and side panels need to come already with the stock cabinet? For examples, if the doors were of walnut veneer, would the same veneer need to be applied to the side panels or it needs to be to already be part of the cabinets?