- 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 if the unit specified is within the distance you set, in yards up to 40. Distances are only known as a range, so it matches only when the far end of that range is within your distance.
Why you might use it
Checks that a unit is within a distance in yards, up to 40. Use it for abilities with a short radius, such as Thunder Clap, Consecration, Frost Nova or Arcane Explosion, only when the target is close enough to be caught. Distances can only be measured as ranges, and the condition uses the far end of the range, so it is true only when the unit is surely within the distance. See also Beyond Melee Range and Distance Count.
Example
your target is closer than 10 yards
Settings
What each setting in the condition editor means, from the game's own help.
Unit - The unit this condition uses as the target of comparison.
Distance - This distance Unit must be within, in yards. There are significant restrictions on how accurately distances can be measured, and all distances are within a range. This will assume the unit is at the maximum of the measurable range. Only distances less than or equal to 40 yards can be measured.