The Upgrade Service is limited to the technical conversion and adaptation of the database to make it compatible with the Target Version, the correction of any Bug directly caused by the upgrade operation and not normally occurring in the Target Version, and the conversion of the source code and data of Covered Extra Modules for the Target Version.

Extra Module - An extra module is a directory of source code files, or a set of Python-based customizations created in a database (e.g. with Odoo Studio), that adds features or changes the standard behavior of the Software. It may have been developed by the Customer, by Odoo SA, by an Odoo Partner on behalf of the Customer, or by third parties.

Covered Extra Module - A Covered Extra Module is an Extra Module for which the Customer chooses to pay a maintenance fee in order to get support, upgrade and bug fixing services.

From https://www.odoo.com/documentation/15.0/legal/terms/enterprise.html#upgrade-services