This closes one or more native files.  Y is a scalar or vector of negative integer tie numbers.  The files associated with elements of Y are closed.  Native file untie with a zero length argument (⎕NUNTIE ⍬) flushes all file buffers to disk - see File Untie for more explanation.

The shy result of ⎕NUNTIE is a vector of tie numbers of the files actually untied.