It uses FIDEs official rating rules and includes the new 400 rating difference rule used in the ELO rating system (most apps do not calculate correctly with this rule) You can enter game data and the rating calculations are automatically saved, so you do not need to re-enter them if you add further games later on. You can save a rating list as a tournament to keep track of your results and improvements from chess tournament to chess tournament.
There is also a graphical overview of your rating improvements from tournament to tournament. The app can also keep trak of your performance in the various openings.
Finally, you can backup your rating info to the cloud and sync it across different devices. Useful if you own both a phone and a tablet and want to have access to the same info.
Furthermore, various k-factors are supported - also non-FIDE k-factors used by many national chess federations, so you can use this app for local tournaments as well.