The term man-at-arms primarily denoted a military function, rather than a social rank. The military function that a man-at-arms performed was serving as a fully armoured heavy cavalryman; though he could, and in English armies often did, also fight on foot. The man-at-arms could be a wealthy mercenary of any social origins, but more often had some level of social rank based on income, usually from land.
The terms knight and man-at-arms are often used interchangeably, but while all knights certainly were men-at-arms, not all men-at-arms were knights.
Pewter Medieval Knight (on Horseback 13C) Suit of Armour # 7658
Pewter Medieval C15th. Knight with Pennant 4.25ins. Ancestors # 7706
Pewter Medieval C16th. Mounted Knight 4.25ins Ancestors # 7689
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Pewter Medieval Knight on Horseback with Lance 4.25ins Ancestors # 7729
Pewter Medieval Knight with Axe & Crest 4.25ins. Ancestors # 7705