This tag refers to Compost Yukkuri, a yukkuri who's made to eat organic trash so that it can process it into bean paste. A yukkuri counts as a compost only if an anon specifically makes it eat garbage to process it, or if an anon specifically calls it a compost (or any composter synonym like garbage processor). If an anon makes a yukkuri eat garbage only for abuse it does not count.
Generally compost yukkuri is obtained from picking up random yukkuri from the streets or buying a cheap yukkuri from a store. Some stores also sell yukkuris genetically engineed to be composts. Sometimes a pet yukkuri who turns shithead is also demoted into a compost as punishment.
Usually used as a form of abuse. Most yukkuri believe that being adopted by a human means becoming a pet yukkuri and as such will be given sweets and an easy home. Thus these yukkuri will feel confused or angry when they found out that they're made to eat garbage and live in a trash can.
If a compost refuses to eat garbage then one way to push it is by removing it's accessory. This will break it's self confidence and makes them more willing to eat garbage. Some anons also take this a step further by owning actual pet yukkuris to make them feel even more worthless in comparison. This can risk the yukkuri developing anti-yukkuritis but as compost yukkuri are disposable and easily replacable it's often not a problem.
That said not all compost yukkuri are abuse victims. Some yukkuri are content or even willing to become compost. Becoming a compost guarantees that they have a steady supply of food and a house. Sure, garbage and a trash can are much worse than the sweets and house obtained from being a pet but it's often better that the living conditions of a gutter trash. Plus being a compost benefits from not having as much restrictions as a pet yukkuri, for example composts usually aren't forbidden from reproducing (unlike pets) as long as they're content with their children also being composters. Anko467 is a good example of a happy compost yukkuri.
NOT to be confused with Yukkuri Compost, where the yukkuri(s) itself is the one being processed into compost.