Thanks, Katie. I see that the external ID datatype does not work as
planed. At least I thought the original idea was to clean up the UI by
moving hard-to-understand string IDs to a separate section. From the
discussions on these pages, I see that the community uses criteria that
are completely unrelated to UI aspects, but have something to do with
the degree to which the property encodes a one-to-one mapping. I guess
this is also valid, but won't be useful for UI purposes. I will need to
use another solution for my case then.