A term used to denote a personal bodyguard or attendant for an important noble serving in a Dragaeran army.
A chainman is presumably responsible for maintaining the weapons and armor for his commanding officer, as well as assisting him in girding himself for battle.
A chainman may also be called upon to perform other special services for his lord, and presumably reports to him directly, without being directly subordinate to any other members of the leadership of the army in which he serves.
An example of a chainman is Molric e'Drien, who served as chainman for Adron e'Kieron in his Breath of Fire Battalion. Generalizing from this one example, it seems likely that a chainman is typically a close relative or friend of the lord he serves.