Allowed Values for Topic data and the role of Project Extensions

I'm not sure how to handle Project Extensions for allowed values on topics (3.1.4).

We had some discussions about it in our team but didn't get to a solution as the standard is very vague about what should happen if values are posted to the server that do not appear in the lists of `topic_type`, `topic_status`, `priority`, etc.

Are these lists just non-mandatory suggestions for the user? Or should these be enforced if defined? I think the second option makes more sense, given that the lists can be narrowed down even further by Per-Entity Authorization (1.2.1). (Or could they also be *extended* by Per-Entity Authorization? Or defined completely different? I don't know.)

In any case, what happens, for example, when one tries to import a BCF-XML file that contains different values? As long as our workflow was solely file-based, we offered a mapping table for such cases. This worked OK. But such a mapping can not currently be shared with a BCF server via a defined BCF-API interface, so every client would do its own mapping.
And it still doesn't answer what should happen with values that do not fit even after the mapping. Should such topics be rejected? Or accepted with offending values changed to defaults? Speaking of which, there doesn't seem to be a possibility to even define default/fallback values. Of course you could always just use `null`, but is `null` a valid option if a list of allowed values exists? Would a topic without a Type or Status even be valid? BCF-XML certainly doesn't think so:
```XML

```
BCF-API disagrees:
```JSON
"topic_type": {
"type": ["string",
"null"]
},
"topic_status": {
"type": ["string",
"null"]
},
```
Which leads to a bizarre situation: With BCF-API, you can can have topics that do not contain sufficient data to be exportable to BCF-XML. This means no less than BCF 3.0 being incompatible with BCF 3.0. What the hell?

I'd appreciate if you could flesh out the documentation to make the intentions behind these concepts clearer.