From top to bottom
Top: Empty
Middle: With food
Bottom: Dirty
When the eat regime is active (or just before), a cook will put clean food trays on a serving table in a canteen, along with food. Prisoners will take one food tray each, along with cooked ingredients. When they have finished, they will leave a dirty food tray, which will be washed in a sink by a cook to reuse.
Food trays cannot be sold, only dumped. The amount of food trays is controlled automatically: if there is not enough of them, more will be bought. Sometimes their amount goes beyond prisoner capacity.
Extra food tray packs can be bought in the objects toolbar in packs of 40.