Special Characters in Authority Identifiers

Is there a standard system for representing special characters in authority identifiers? Most of time I see them replaced by the most similar-looking standard character, e.g. a for both ä and å. Several times, however, I've come across an identifier that uses one or a combination of characters that (I assume) better represent the missing character phonetically. (In my example, probably oa for å and maybe e or ae for ä.) Is there an official list, or is either tactic considered more correct?