Command is persuasion by force of presence — the assertion of authority, and the threat, spoken or otherwise, that stands behind it. It is what a character tests to make someone do a thing they would not do naturally or willingly: hold a line, open a gate, put a weapon down, answer a question they would rather not.
It is the bluntest of the persuasion skills and the fastest, and it costs something every time. People obey a command and remember having obeyed it.
Resistance. A target is usually permitted a Will test against the commander’s Command. Where the pressure is direct physical intimidation rather than authority — the threat implicit in the person making it — the target tests Initiative instead, because what is being asked is whether they hold their nerve.
Command also directs troops once a fight has begun, and a captain’s ability to control a formation in contact is generally more valuable than their ability to inspire it beforehand.