Language has incredibly subtle nuances that mean different words change meaning in various ways, and the ways in which they change it vary between speaker and listener. Unless the speakers of a language decide that the nuances aren't worth the effort of maintaining more than one word for the same thing, in which cases, less favoured words die out. You used the word "isotonic" instead of "equivalent" or "synonymous", presumably for good reason.

In my mind, "code" implies something opaque, mysterious, and difficult, rather than something which is open to all.