elisp: Byte-Code Type

 
 2.3.16 Byte-Code Function Type
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 “Byte-code function objects” are produced by byte-compiling Lisp code
 (SeeByte Compilation).  Internally, a byte-code function object is
 much like a vector; however, the evaluator handles this data type
 specially when it appears in a function call.  SeeByte-Code
 Objects.
 
    The printed representation and read syntax for a byte-code function
 object is like that for a vector, with an additional ‘#’ before the
 opening ‘[’.