Any troops skirmishing would kneel as the default.
Any troops who habitually fire in three ranks (essentially most nations up to 1808) will have the front rank kneel to fire. (The use of the third rank as skirmishers was a tactical innovation that grew out of the disastrous 1806 campaign.)
Any troops who form square (as opposed to battalion mass) to repell cavalry, will have at least one rank (two for the British) kneeling.