A unit describes something you want to do within systemd and its desired state. The desired state can be either:

* `inactive`
* `loaded`
* `launched`

Fleet works out how to go from the current state to the desired state.

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Another important aspect to a unit is the Machine ID. This is largely handled by Fleet based on criteria given in the `X-Fleet` section of the unit options.

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To start a unit you would request the desired state to be `launched`.