Single/double character domains: document exceptions?

[Enrico204]

To be 100% transparent, maybe an explanation why some exception are made should be added to the repository. For example:

• a
• rf
• sm

Why these domains exists, given the restriction published (Name-Restrictions)? Given these, why others (like #74 or #122) are rejected?

Edit: I'll add also the case of rep.o (#133) which was rejected because it's "reserved" but it's not in the Wiki page of restrictions

[Enrico204]

@jonaharagon can you dedicate few minutes to this issue?

[midn1]

Would like an answer to this, too.