Thank you for your answer, it was really helpful.

But all the data you tried to show in the receipt come from the front-end (customer info for example).
In my case I wanted to show a new field which gets its value on the server.

I don't know if this is the best solution but I've overridden the create_from_ui method in pos_order.py, and the _save_to_server method in pos_store.js so that i add the new field to the response.

The user need permission to access res.config.settings