Odoo version
What changes in the connector's behaviour at each version, and why the right number matters.
The number genuinely changes behaviour
The Odoo version is not a display-only detail: it changes which fields the connector writes to and which features appear for you. Left empty, it is assumed to be 16.0.
From 17.1
Odoo renamed the field that records a returned quantity on a stock move. The connector picks the right field from the number you entered, so a number lower than reality makes stock returns fail.
From 19.3
The ZATCA e-invoicing features appear: the Automatically Send Invoices to ZATCA switch on the Accounting tab, the ZATCA Invoice Error and ZATCA Credit Note Error screens in the sidebar, and the ZATCA badge on the order row.
On any earlier version those screens are not in the sidebar at all.
Enter the real number
Do not enter a number higher than your actual version hoping to unlock features: the connector will try to write to fields that do not exist, and whole steps fail with obscure messages.
Where to find your version
In Odoo, open Settings then About, or look at the bottom of the login page. It shows as something like saas~19.3 or 17.0.