I usually create a new Field by using the Debugging Mode and then clicking "Edit FormView" when I am on the appropriate form and writing into the code something like:
<field name="x_delivery_date"/>.
Also I can show it afterwards on the printed report like this:
<div name="x_delivery_date" t-if="doc.x_delivery_date">
<strong>Delivery Date:</strong>
<p t-field="doc.x_delivery_date"/>
</div>
But how do I display a field (commitment_date), which is available in the model (sale.order) in another models formview (account.invoice)? I guess that I have to use object relations or related field, but I don't know how. I hope somebody can help me. Many thanks in advance.