The idea is to cause the sensation of soiling (peeing or pooping) a diaper whether you are wearing one or not.
I would suggest adding a sense of pleasure during this and and a much stronger sense of pleasure and happiness when done in real life.
This thus could be a trainer file for those are unsure or can't deal with the mesh but want the pleasure of meshing one.
Obviously there could be many variations to this idea such as a single episode of messing continuous messing or repeated messing over a period of time.
This idea could also be made slightly malicious if continuous messing in that the subject might be able to tell when s/he needs to actually go.