Game mode ID

Game mode ID is an interface system that indicates the player's current game mode, format, name, rank and other information, through special icons shown in the bottom left corner during matches.

Play mode
Play mode matches feature the corresponding icons from the Play screen, including indication of Standard or Wild format. Ranked matches also feature the player's current rank, and are shown for both the player and the opponent.

Adventure mode
Adventure matches display the current adventure's icon. Practice mode is an exception, and uses the default name-only display. All adventure mode matches also show the opponent's difficulty (Normal or Heroic for adventures, Normal or Expert for Practice mode matches).

Arena
In Arena the icon also shows the player's total number of wins and losses so far. This is only shown for the player, and not the opponent. It is also visible by spectating players, but not by players on the friends list.

Tavern Brawls
The icon for Tavern Brawl matches can be moused over to reveal the description for the current Brawl

Trivia

 * The original player name display was very limited, and in most game modes displayed only the player's name. The system was upgraded to 'Game mode ID' in Patch 5.2.0.13619 and significantly expanded. The new game mode ID system was designed by senior interface designer Max Ma, and first teased in a video shared on the official Twitter account, inviting applicants for a vacancy in the UI department.

Patch changes

 * Expanded to the new 'Game mode ID' system:
 * Icons for Casual Play mode added, including indication of Standard or Wild format
 * Icons for adventures added
 * Icon and mouse over information for Tavern Brawls added
 * Icon and win:loss information for Arena added