According to the Buddhist texts: After coming back from alms round, we should put the extra food we will not eat aside for the bhikkhus who come later and need it. They too can take from or donate to this bowl. It is quite useful and is used everyday by many monks. It reduces much waste and makes pre-leftovers edible, allowable and shareable.
Monks cannot eat food that has not been personally offered by a donor. This food has been made allowable because it was offered during alms round. Novices and lay people cannot touch "already-offered-and allowable-food" unless a monk holds the edge of the bowl. If they do, it will need to be offered again because they will be assumed as the new owners. In this photo, some young novice monks who want some of that extra food ask a bhikkhu to touch the bowl as they take from it.
Photos by Yogi Bram (Belgium)