I knew all that already, I mean is if there is any difference between articles that cover the same area and are made of the same material. Dresses and robes both have max coverage and [UBSTEP:MAX] + [LBSTEP:MAX], but robes weigh slightly more and have double the [LAYER_PERMIT] value.
So would it be better to wear multiple robes or multiple dresses? You could wear more dresses than robes and if their protective value per article is the same, dresses would be superior to robes.
But the [LAYER_SIZE:xx] value is different for them and I suspect that plays into how 'thick' or hard it is to penetrate. Which would be important in determining the absolute optimum types and amounts of clothes to wear.
[LAYER_SIZE] plays into how many of that item you can wear over an item with a given [LAYER_PERMIT] value. In this case, because a dress is [LAYER:UNDER] and has permit and size values of 50 and 10, respectively: you put on a dress, which [LAYER_PERMIT:50] value of which allows five more [LAYER_SIZE:10] dresses to fit over it. Now the UNDER layer is full, and you move on to the next layer, [LAYER:OVER], where you are applying your robes. Robes have [LAYER_SIZE:20] and [LAYER_PERMIT:100], which you'll notice would work out the same way as the dresses; wear one and it allows five more. However, since you already are wearing six dresses, the total upper body armor size is already 60; so you can only wear three robes over the six dresses, bringing you to a [LAYER_SIZE] total of 100, which is the max [LAYER_PERMIT] for robes. If you wanted to change the ratio, you'd have to remove two dresses for each extra robe you wanted to wear, which given that they cover the same area and dresses are lighter, would be a bad idea overall.
Then of course you can put three cloaks over the top of that ([LAYER_SIZE:15], [LAYER_PERMIT:150]) for a maximum total of twelve layers of clothing.
This section on the wiki explains the concept in a different way than I did, which may or may not be easier for any given person to understand.
As far as I know, the various tags within the armor raws do not effect toughness in any way, aside from [STRUCTURAL_ELASTICITY_WOVEN_THREAD] dictating that they are flexible items rather than rigid or semi-rigid as with proper armors. And material of course.