Thanks for your suggest.
Currently, Users using, for example, TruePhone, can add custom fields to their contacts.
These do not seem to get to DavX or else are not passed over by DavX.
Yes, DAVx⁵ can only synchronize fields it knows.
a) represent Nextcloud fields such as Language, Relationship, Geo, Timezone, Deathdate etc (most of which are RFC standard) on the Android handset.
b) sync the custom fields created in Android apps that allow it, so at the very least the data is retained in the vCard, if not displayed at the server end. I assume these properties are stored by the relevant apps in a custom way…?
It would be possible to store such fields in the Contacts database, but with a proprietary (custom) MIME type. As I see it, the most difficult part is that DAVx⁵ only stores/retrieves the data, while the Contacts apps provide UI. So implementing this would require cooperating with Contacts apps. Do you know Contacts apps whose developers would be interested in that?
Also, just for curiosity: What are your most needed additional fields and what is your use case for that?