The below requirement is listed in the article on requirements for public apps. I've read through the entire API Terms and the more detailed document that is linked from it, and I am still not quite sure what is expected from our app
Basically we offer our users a UI inside of our platform where they choose which fields on the customer resource they want to import into our platform. This is then parsed from webhooks in real-time as customers are created, and on "full syncs" which run on the user's demand where we download all customers (after creating mappings for the first time for example).
The linked document states that it should be written through the customer resource, so the only interpretation I can arrive at is that if I collect the first name of a customer, then I must write back the first name of the customer. However, I am certain that this is not what Shopify expect of me in this case because it would serve no purpose if I always wrote back any field I've read, but I can't come to a different interpretation for it and partner support on the phone are not sure either.