⎕AV is a deprecated feature and is replaced by ⎕UCS.
This is a simple character vector of all 256 characters in the Classic Dyalog APL character.
In the Classic Edition the contents of ⎕AV are defined by the Output Translate Table.
In the Unicode Edition, the contents of ⎕AV are defined by the system variable ⎕AVU.
⎕AV[48+⍳10] 0123456789 5 52⍴12↓⎕AV %'⍺⍵_abcdefghijklmnopqrstuvwxyz¯.⍬0123456789⊢¥$£¢ ∆ABCDEFGHIJKLMNOPQRSTUVWXYZý·⍙ÁÂÃÇÈÊËÌÍÎÏÐÒÓÔÕÙÚÛ Ýþãìðòõ{€}⊣⌷¨ÀÄÅÆ⍨ÉÑÖØÜßàáâäåæçèéêëíîïñ[/⌿\⍀<≤=≥>≠∨∧ -+÷×?∊⍴~↑↓⍳○*⌈⌊∇∘(⊂⊃∩∪⊥⊤|;,⍱⍲⍒⍋⍉⌽⊖⍟⌹!⍕⍎⍫⍪≡≢óôöø"#&´ ┘┐┌└┼─├┤┴┬│@ùúû^ü`∣¶:⍷¿¡⋄←→⍝)] §⎕⍞⍣%'⍺⍵_abcdefghijk