The ‘sets’ icon showing 3 small circles applies to two types of coin:
Any of the older ‘sets’ coins that are found in circulation are coins that have been broken out of their packaging and spent at some stage! It doesn’t happen often, but it is technically possible and for this reason the ‘sets’ coins are included within the app by default. Newer ‘sets’ coins found in circulation may be due to them recently being circulated for the first time (once a few confirmed sightings are made, the ‘sets’ icon is removed).
If you’d rather strictly stick to collecting coins that were or are currently known to circulate, the ‘sets’ coins can be disabled and rendered invisible using the option in the settings (cog icon) options screen.
The Check Your Change admin is Mr C H Perkins, publisher of numismatic publications in printed and eBook format. Author of "Collectors' Coins - Decimal Issues of the UK" and other books on British coins and related subjects.