ArenaSeason.InProgress = 0 disactive the arena season, so a player will not receive the arena points through the 10 games (weekly), and can't see the arena team in his honor/arena window status.
ArenaSeason.ID handle which arena season is active, the difference between the season are the set pvp sold by the pvp/honor vendors, the set that these vendors sell depend from arena season, so if you change this value the items of these vendors should be change.
Edit: I'm not sure about the last but the arena season are strongly connected with the pvp sets