Bossbar

A boss bar is a bar that is shown when you use the  command, or for the wither, look at the wither's direction.

Showing a boss bar
A boss bar could be shown by using the  command.

Color
The boss bar has colors depending on which boss mob is active.

The ender dragon has a color of pink. The wither has a dark purple or pink boss bar.

ID
A boss bar has an ID, which can be used to do certain actions with one boss bar, including (but not limited to) disabling one.

Disabling a boss bar
A boss bar can be disabled by using.