Byte-code function objects are produced by byte-compiling Lisp code (see Byte 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. See Byte-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 ‘[’.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Byte_002dCode-Type.html