Score! Match pairs users for multiplayer games based on their Arena. In addition to this, the game server ‘ping’ is taken into account such that the game experience is acceptable for both players. We also consider whether a player is a new player (fewer than 20 matches) to reduce matching new players with very experienced players. Finally we consider each player's previous matches to avoid matching with the same player repeatedly.
Things that are not considered during matchmaking are: Team Rating, Stars, Win Rate or Streak, any sort of ELO/Ranking/XP score, anything related to player spending.
This means that in general users should match with another player in the same Arena, but it is possible to match with a player from a different Arena, but this would only be due to lack of suitable available players from the same Arena online at the matching time.
There is no control during matching to manage the difficulty experience of individual players, but as players move between Arenas it is expected that opponents will be more or less challenging as a result.