- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks whether the role of the unit specified (tank, healer or DPS) is the role you choose. Where the editor offers no unit, it checks your own role.
Why you might use it
Checks a combat role (Tank, DPS or Healer). On Retail it reads another unit's role, such as whether your target's target is a tank, so a DPS can avoid stealing a mob from the tank or a tank can Taunt when the mob turns to a healer. On Classic progression (Mists) and WoW Forever it reads the role of your own active talent group instead, which makes it a natural switch condition between a tank, healing and damage rotation. It is not available on Classic Era or TBC Anniversary; on Retail it is unavailable when the game hides the unit's identity. See also Specialization.
Example
your role is Tank
Settings
What each setting in the condition editor means, from the game's own help.
Role - The role set for your active talent group. (only on Classic progression, Forever)
Unit - The unit this condition uses as the target of comparison. (only on Retail)
Role - The current role of Unit. (only on Retail)
Tank - Designed to keep the attention and take most of the hits from enemies. aka. Meat Shields.
DPS - The primary damage dealers against enemies, and lovers of standing in fire.
Healer - Those who keep everyone else alive by healing them. Thank them!