However, the number of available fields seems to be related to the account type. Also with the standard android contacts app, I’ve got far less fields to add with a DavDroid account then with a local phone or google account.
So my assumption was that DavDroid could indicate which fields it can synchronize and that only those fields are available in the editor.
That’s correct, these definitions can be found in https://github.com/rfc2822/davdroid/blob/master/res/xml/contacts.xml
But it’s worth to notice that there are limits set by the various Android versions; for instance, if you remove the
sip_address, Android 4.0 will crash (there are hardcoded assertions which will fail).