From the back of the book:
Feel the thrill of melee, brace against a monster's charge, master the martial arts, let your arrows fly -- do battle with the best combat system around! This system is one of the four cornerstones of the Rolemaster Standard System. However it is fully compatible with the earlier editions of Rolemaster.
In Arms Law, you get:
A fantasy/medieval melee and missile combat system including tables for 29 different weapon types (including attacks and fumbles).
Thirteen attack tables that handle special types of attacks, including animal-type attacks, falling damage, brawling, and martial arts.
A dozen different critical strike tables that detail severe damage delivered from critical hits -- including Slashing Critical Strikes, Puncturing Critical Strikes, two types of Martial Arts Critical Strikes, Unbalancing Critical Strikes, etc.
Guidelines for using these tables with other FRP games.