Basically where you could layer clothing over/under armour or vice versa. This can allow you to really mix/match things and find a unique look that you like.
you could have tight fitting clothes which could be worn under armour and things, which can be seen in places where the armour doesn't cover. Then you can have looser clothing that could be layered over that armour, such as Tabbards, cloaks, etc.
This way if you play characters like say, a warrior that has to wear armour, you can still have a more unique appearance then looking like every other warrior with that plate armour. Throw on a tabbard with a coat of arms/colouring that you like, maybe wear a cloak on the back of it or not. This allows flexiability and customization that is not that hard to do and can offer some different choices as far as finding an appearance you like for your character.