string manipulation – Purpose of ToCharacterCode

The result is not decimal: it’s a List of Integers that by default displays as decimal in {}.

The encodings are mostly Unicode, which has the same numbering as ASCII for characters in ASCII, but includes many more characters:

ToCharacterCode("Aあ")
(* {65, 12354} *)

There are some exceptions for backward compatibility, reflecting the fact that Mathematica is older than Unicode.