⎕ML determines the degree of migration of the Dyalog APL language towards IBM's APL2.  Setting this variable to other than its default value of 1 changes the interpretation of certain symbols and language constructs.

⎕ML←0		Original Native Dyalog
⎕ML←1	Z←∊R	Monadic '∊' is interpreted as 'enlist' rather than 'type'.
⎕ML←2	Z←↑R	Monadic '↑' is interpreted as 'first' rather than 'mix'.
	Z←⊃R	Monadic '⊃' is interpreted as 'mix' rather than 'first'.
	Z←≡R	Monadic '≡' returns a positive rather than a negative value, if its argument has non-uniform depth.
⎕ML←3	R←X⊂[K]Y	Dyadic '⊂' follows the APL2 (rather than the original Dyalog APL) convention.
	⎕TC	The order of the elements of ⎕TC is the same as in APL2.

Subsequent versions of Dyalog APL may provide further migration levels.

      X←2(3 4)

      ⎕ML←0
      ∊X
0  0 0
      ↑X
2 0
3 4
      ⊃X
2
      ≡X
¯2

      ⎕ML←1
      ∊X
2 3 4
      ↑X
2 0
3 4
      ⊃X
2
      ≡X
¯2

      ⎕ML←2
      ∊X
2 3 4
      ↑X
2
      ⊃X
2 0
3 4
      ≡X
2