Extend data model to allow for custom fields with biber in tool mode

You are correct in your assumptions. The field elements describe allowable fields. Biber uses this to decides whether to allow the field. Other aspects of the datamodel are used for validation. The .conf file datamodel format is not really documented because people rarely use custom datamodels in tool mode. The biblatex macros for datamodel manipulation write the XML you see to the .bcf file (but with a bcf: namespace).