It's an extension. Since it's undefined by both APL2 as well as ISO 13751, it's a compatible extension, and a quite useful one at that. For example, creating a multiplication table is:
×/¨⍳10 10, which also extends naturally to more dimensions, unlike the alternative which uses
∘.×.
There was a discussion about this on this mailing list last year or so when the extension was added. My understanding was that the extension was taken from Dyalog, but you say it's not there, it must have been taking from somewhere else. Perhaps it was from NARS2000?
Regards,
Elias