I'm unsure if this is intentional or a bug, but when creating business partner relationships, you can't select an address to be a ship-to address. The checkbox is there, but it's marked as not updatable (unlike IsBillTo, IsPayFrom, and IsRemitTo) under tab and field, so there's nothing you can do with it.
Is this intentional? Is this a bug? Woudl marking it as updatable break any future updates?
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Probably a dictionary error - see the last two comments here:
https://idempiere.atlassian.net/browse/IDEMPIERE-362
I think it must be read-write