Doesn't it depend on region? ISTR they don't use ß in Switzerland, so there is no Massen versus Maßen.

Actually, it's more messed up than this. In Germany, the DIN 5007 defines two variants: 1) ä and a are equivalent (normal order, e.g. in an encyclopedia) 2) ä and ae are equivalent (to be used in "name lists", e.g. telephone books)

But Germany is not the only place where German is one of the official languages. E.g. there is an "Austrian order", where a is followed by ä, except for name lists where often DIN 5007 variant 2 is used as well (ä equivalent to ae).

Then again, Swedish also knows the letter "ä", but there ä is (usually) sorted after z (well, after å which comes after z).

Now, imagine you run an English-language service but need to sort surnames such as "Ährlich". What sort order would be the right one? DIN 5007/1, 5007/2, Austrian, Swedish, something else entirely? Imagine you run a German-language service that is frequented by Germans and Austrians...