I dont know why people are getting so worked up about the sticker thing. Its just a simple little thing that can be stuck on the relevant page(ratings or whatever) in someones logbook that shows what the nature of the differences training was and the date it was completed. A quick reference guide if you like. Saves having to trawl through pages of logbook entries to find the relevant training flights and Instructor endorsement. I always use them if I carry out differences training with someone.