Microsoft created it, it's a numeric constant in c-code, it's sitting on your hard drive (most likely). 23 refers to a combination all 4 value types (constants) of Special Cells -> Formulas. Those being:

xlErrors, xlLogical, xlNumbers, xlNumbers.

All of which are early bound references numeric, constant arguments. E.g., xlNumbers = 1.

23 is actually the sum of all four numeric constants:

Errors = 16
Logical = 4
Numbers = 1
Text = 2

There's only one argument to be passed. 23 is moot in this case as the argument is both optional and 23 is the value by default unless stated otherwise.

You may also want to refer to the SpecialCells Method in the VBE help file.