The Commodities Transfer Dialog
The commodities transfer dialog lets you transfer commodities from the unit to the city it's in.  It can be brought up either by Command Clicking a unit in the Stack List when that stack is in a city or from the 'Load'  button in a units Army Info Window.  The left column is what is on the unit, the right column is what is in the city.  Another way to get commodities on and off units is the 'Quickload' and 'Unload All' menu commands.  This is an old picture. New Commodities Transfer Dialogs will look different but function the same.