The Command spell states:
You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn.
If there were two doors and you pointed at one and gave the Command, "exit" - are there any guidelines as to whether the gesture part of the Command clarifies the command or does the creature get to choose how to interpret the command?
Before I start, the spell does explicitly say that the DM determines how the target behaves, so it's ultimately the DM's call. As I read it, gestures are not part of the magic of the spell, but they might be part of the mundane context of the command.
First, spells only do exactly what they say they do, and no more. If gestures were part of the magical command, they would have been included in the text of the spell. Specifically, the one-word limit reduces the spell's versatility--adding gestures basically bypasses that limitation.
Second, the spell only requires that you can see the target, not that the target can see you. If you try to use gestures to Command a creature that can't see you, does it still work? Nothing in the spell text says that the creature can somehow understand a gesture it can't see, because gestures are not part of the spell.
However, creatures have to use mundane, nonmagical context to interpret the command that they're given. For example, "approach" means that the creature has to locate you and figure out how they're going to move closer to you, and "drop" means the creature has to identify whatever they're holding and drop it.
If you issue the command "exit," and point, the creature may use the information you give it in order to carry out its command. However, they are not magically compelled to do anything other than strictly follow the single word command, so they might exit via other means, like teleporting away.
The DM chooses how to interpret the command should you use any command other than the ones written within the spells description.
You might issue a command other than one described here. If you do so, the DM determines how the target behaves.
No
The command spell cannot be clarified with any cues. It gives a single word command, that will be carried out in the next turn, if possible. If you need more nuance, then the right spell is Suggestion. That both allows clarification and has a longer duration.
The "exit" command, you suggested would make the creature to move towards the nearest exit. In a situation with two doors, it would exit the room through the nearest door, assuming the door was reacheable in a single turn.
