From the back of the book:
"For nearly 300 years, war has raged across the Inner Sphere as the rival Successor States fought one another and fearsome invaders called the Clans. This maelstrom of conflict has created untold suffering and destruction -- and a never-ending demand for the services of mercenary soldiers.
BattleTech Field Manual: Mercenaries provides an in-depth look at mercenaries in the 31st century, from the inner workings of a mercenary unit to detailed profiles of more than forty prominent mercenary bands. This how-to manual for would-be soldiers of fortune includes the histories, officers and favorite tactics of select mercenary units. special rules that reflect the unique capabilities of each mercenary command, and rules for creating and running your own BattleTech mercenary units."