- 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 how far the unit specified is through its current channel, from 0% at the start to 100% at the end, compared with the percentage you set. It does not match when the unit is not channeling, whatever the comparison.
Why you might use it
Compares how far through its current channel a unit is, from 0% to 100%. Ticks are evenly spaced, so tick N of a K-tick channel lands at N/K whatever your haste: to clip Mind Flay (3 ticks) after its second tick, use at least 66%. It is false when the unit is not channeling, and unavailable when the channel's times cannot be read, as can happen for other units on Retail and WoW Forever. See also Channel Time Remaining.
Example
your channel progress is greater than or equal to 66%
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.
Operator - How to compare Channel Progress with the Progress value. This comparison uses basic mathematical comparison operations.
Progress - How much of the current channel of BLIZ_YELLOWUnit is done, from 0% when it starts to 100% when it ends. Ticks are evenly spaced through a channel, so tick N of a K-tick channel lands at N/K whatever your haste: to clip Mind Flay (3 ticks) after its second tick, use at least 66%. Pushback shortens a channel and the progress follows it. If the unit is not channeling, this condition is not successful, whatever the operator; if the channel's times cannot be read, it is unavailable, including under NOT.