It's not sort. In Kap, sort down is ∨. The grade functions returns a sorted index. Basically ⍒ 3 1 2 1 1 returns 0 2 1 3 4. So yes, it performs a sort, but it returns an index that can be used to later look up the respective elements.

You can read more about it here if you're interested: https://www.aplwiki.com/wiki/Grade

And here's a silly example where a random array is created, then an array lookup is performed on the sorted indexes:

https://kapdemo.dhsdevelopments.com/clientweb2/#a%20%E2%86%9...