Since our rating system is based purely on scores, it does not take into account defaults or walkovers. However, these results are still displayed on the player's profile page along with their other scores.
With regards to withdrawals, if a player withdraws either a) before the match starts or b) during the match but before either player wins at least four games, it does not factor into the UTR Rating's algorithm. It will, however, be displayed on the player profile. This will hopefully be a disincentive for the small subset of players who withdraw quite frequently. We are aware of and monitoring junior players abusing the system.
While incomplete or unfinished matches are used for rating calculations in cases where at least four games were won by either player before the match was stopped.
Note: To know which matches are contributing to the rating, look for the gray tennis ball icon on the right-hand side of the match result. This icon allows you to easily see which matches are currently contributing to your UTR Rating today and is updated daily. 
For more information on the withdrawals analysis, click here.