Size, Permit, and layering armor
The Size and Permit values govern how much clothing or armor can be worn.
Under the old system the lowest "permit" value for any given body part is used: for instance, if a dwarf is wearing a dress (permit value: 50) and a total of 50 or more size worth of clothing on the upper body, it cannot put any more clothing on the upper body. (This explains why the old dungeon masters tend to wear several cloaks: they arrive at the fortress wearing only a cloak on the upper body (permit 150), and can put on a total of 10 of them, at 15 size each.)
Unfortunately, [LAYER:COVER] items are the only items playing by the old rules. This much is certain from testing in arena mode.
* If the item to be add is a [LAYER:COVER] item, add the total item size on the body part, if this sum is less than or equal to the item's permit value then evaluate as true.
* If a [LAYER:ARMOR] item is present or to be added and if the sum of the non [LAYER:COVER] items would be less than the sum of the [LAYER:ARMOR] size+permit values then evaluate as true.
* If one or more items of the same non-[LAYER:COVER] layer as the one being added are present and if the sum of their size values is less than the smallest permit value then evaluate as true.
* If the sum of the size values for all items on the body part are less than or equal to the permit value of the item about to be added then evaluate as true.
The item is allowed if all rules either evaluate to true or are not applicable. This is in addition to the rule allowing only one shaped item on a given body part at a time.
Example: A helm(30 size,20 permit) means you can put on a mask(20,10) or two caps(10,20), but only two head veils(10,100). Any of these configurations can fit 6 additional hoods if desired.
Example: Wearing a cap(10,20) allows only one face veil(10,100), because they are both [LAYER:UNDER], but an additional combined total of up to 9 head veils and hoods is possible.
Note that the armor value of socks and other clothing is unknown under the new system - however, wearing them under "armor" such as boots is recommended for an adventurer.
Adventurer mode follows the arena rules so it is possible to have three chain mail shirts(15,50), a breastplate(20,50), and 25 capes(10,300) on ones upper body plus two caps(or one mask), a helm, and six hoods on ones head. Confirmation is needed to see if fortress mode follows the old rules or the new arena rules.
Source:
http://df.magmawiki.com/index.php/Armor#Size.2C_Permit.2C_and_layering_armor#ixzz0vwHaLB9i