It is an extendion to the Weather and Sky Pack.
It randomises the clouds so they will look different each time you start a scenario that uses the pack.
So instead of one “high cloud” pattern, a script will randomly choose from many high cloud patterns.
So it is indeed built into the Sky And Weather pack. But you will have to pay, and that is cool enough. You also had to pay for the Frankfurt Highspeed route extension.
And i will take unofficial credit for suggesting this functionally to AP already back in 2019 and again in 2021. So at least they could have credited me with coming up with part of the concept for this.