[##:##] Command : lunload <COMMODITY> <UNITS> <AMOUNT>
[##:##] Command : lunload plane <UNITS> <PLANES>
The lunload command is the opposite of lload and follows the exact same syntax ... so why look here for documentation?
The only thing to know is that lunload won't unload more of a thing if the sector already has 999. For example, if a unit has 10 food, and the sector has 994, you can only unload 5 of the 10 food.