From (scattered indexing)

If the left argument is a singleton, it is opened and its items are indexes along successive axes.

   (<0 2 ; 2 3) { m
 2  3
10 11

What if the left argument wasn't a singleton?

   (0 2 ; 2 3) { m
2 11

What is going on here? Nothing special, as this is just your old friend rank. The dyad { has a left rank of 0 and a right rank of _ . This means that the left argument is taken as 0-cells and the right argument is taken in its entirety. Visually:

(<0 2)  (first left cell)  { m   gives 2
(<2 3)  (next left cell)   { m   gives 11

The result is assembled from the 2 and 11 partial results.

This is called scattered indexing.

   (0 0 ; 1 1 ; 2 2) { m	   NB. scatter index a diagonal
0 5 10