Specialising in any type (size) of shield improves AC by 1 when using such a shield and allows the shield to be counted against two additional attack per melee round. A double-specialisation in a type of shield improves AC by 2 against missile attacks (for a total improvement of 3 against missile attacks and 1 against melee attacks) and allows it to be counted against one additional attack per melee round (for a total of three additional attacks).
Specialising in padded armour or elfin chain improves AC by 1 when wearing such armour. Double-specialising in either improves AC by another 1 (for a total improvement of 2).
Specialising in any other type of armour reduces such armour’s effect bulk by one step when worn and also increases base movement by 3″. A double-specialisation additionally improves AC by 1.