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More seriously, anyone in a command uniform (that isn't the captain) should be considered a "command intern". When people wonder at having Wesley Crusher or Chekov doing things aside from the helm, it's because when you're on the command track, everything you do should go towards making you a better captain down the road. That means having at least some experience with every function on your ship. It doesn't have to be a super in-depth knowledge (that's what the senior staff is for), but it should all go towards making the officer a more rounded individual so they have more experience to draw from when making command decisions.
Once they make captain, if they're a Kirk type leading away teams, then they're function is to figure out what's going on while they solve the Problem of the Day. If they're a Picard type, then they delegate the data gathering to those below you, but at the end of the day they have to take responsibility for the Big Decision of the Day.
The rest should be self-explanatory, but let me know if I should continue.
