I wonder if the raised areas in the suit and around the waist area are somehow some kind of circuitry or connecting points, which is effectively what links the suit to the armour when it's attached? The belt area seems to be a focal point in a lot of the armoured pics.
I've a feeling that the suit itself - what Superman wears, and what Zod and Jor-El wear beneath their armour - is essentially some kind of intelligent bio-suit which allows various appendages to be attached and is adaptable to all situations.
In some respects it's reminiscent to me of the Centurions toy lines from the 80s; characters who wore an intelligent base suit all the time, which they then added various attachments to whenever required.