Man at Arms
Historically, a man-at-arms was a fighter employed by an army, lord, or individual knights. In Amtgard, men-at-arms are roughly analogous to pages, in that they are taken by squires. Although this is theoretically a mentor-student type relationship, there are a multitude of reasons that a squire might take a man-at-arms.
Men and Women at Arms wear a black belt. In some Kingdoms this is further established with silver trim but many places don't require or use it.
Everything about men-at-arms is custom, so there are no hard and fast rules.
The difference between a Man at arms and a page:
Officially, there is no difference, as neither is officially recognized in the Order of Precedence or most Corporas. Some kingdoms define a man-at-arms as some one who fights, while a page is a non-combatant. Some kingdoms eschew one in favor of the other. Some kingdoms allow squires to take a man-at-arms, and a noble to take a page. In some places, children too young to fight are pages, while everyone else is a man-at-arms.