(Screenshot needed)

When selecting a unit, in the upper left a panel shows the unit type, health and ammo (exact meaning of ammo varies from unit to unit), and options for the following

Unit on/off: For unconstructed units, this can stop build packets if you are low on energy. It can be used, for example, to lay out your expected base during the initial setup, so you can figure out where buildings need to go, and then have them turned off until you are ready for them. (Hint: After doing all layout, select a single building of a type, hit select all (default: ctrl-A), and then hit unit off to disable all of them. You can then turn on only the few that you want to start with. Restricting tower construction based on actual energy availability can speed up the first minute.)

For constructed units, it stops everything for the unit. Unit will not request packets, nor do anything.

Default shortcut: T

Armed on/off: The unit will do its function. This is not just weapons; for example, Porters will send packets to a remote location.

Possible reasons to turn this off:
1, have an invasion fleet fill up on ammo before launching (remember to turn it back on when you take off)
2, you may want to have units getting ready for an attack, and not firing yet (Yes, this can happen).

Default shortcut: Y

Resupply on/off: No ammo packets will be sent to the unit. It will still use whatever ammo is currently in the unit. When that runs out, it will stop.

Possible reasons to turn this off:
1, energy shortage.

Default shortcut: U

Ern held/none/want: Toggle the state of the unit's ERN, if any.

Default shortcut: I

(NB: TYUI, the default shortcuts, are next to each other on QWERTY keyboards.)

Avoid Creeper: Checkbox Found only on movable units. Tells the unit to avoid creeper while moving. Unit will spend more time to fly over creeper in the way.

Possible reasons to turn this off: Get to the destination faster.
Potential downside: A large enough wall of creep in the way will destroy it.

There is no keyboard shortcut for this.

After this, there are (potentially) a number of unit-specific options. What to target for some combat units. What resources for Porters. Etc.