What's the standard for XML-RPC fault codes?

In the process of writing a pingback handler for my website, I noticed that the XML-RPC specification doesn't say anything about what fault codes should be defined and what they should mean. So the question is, is there a commonly accepted standard for fault codes that specifies this information? Which fault codes are defined and what do they mean? Both for XML-RPC in general, and for specific applications (like pingback)? I think I know an answer to this but I'd like to get the community's input on it (and I think it's good reference material to have on SO).