Y may be any array. R is the maximum number of levels of nesting of Y. A simple scalar (rank-0 number, character or namespace-reference) has a depth of 0.

A higher rank array, all of whose items are simple scalars, is termed a simple array and has a depth of 1. An array whose items are not all simple scalars is nested and has a depth 1 greater than that of its most deeply nested item.

Y is of uniform depth if it is simple or if all of its items have the same uniform depth.

If ⎕ML<2 and Y is not of uniform depth then R is negated (therefore, when ⎕ML<2, a negative value of R indicates non-uniform depth).

      ≡1
0
      ≡'A'
0
      ≡'ABC'
1
      ≡1 'A'
1 

      ⎕ML←0
 
      ≡A←(1 2)(3 (4 5)) ⍝ Non-uniform array
¯3
      ≡¨A          ⍝ A[1] is uniform, A[2] is non-uniform
1 ¯2
      ≡¨¨A
 0 0  0 1 

      ⎕ML←2
 
      ≡A
3
      ≡¨A
1 2
      ≡¨¨A
 0 0  0 1