Remnants of a Slain God
Underarmor is a slimmer type of armor that can be worn under a layer of armor. You may normally wear more than two layers of armor together. Typically, only two pieces of underarmor, or an underarmor and regular armor may be worn together. When combining an underarmor with other armor, the normal DR is treated as the better of the two. However, damage resistances from the underarmor is applied to the top layer of armor.
For example, a combination or regular steel platemail and chainmail could be worn together. This would give normal DR of 17, but have piercing resistance of 5 and crushing and slashing resistance of 3.