Hi,

When extending in Js, instances from the parent class remain untouched, but instances from the new child class will have the extended features.

When you include, you are adding the new features to the prototype of the parent class.

Here is a link to the blog which explains in detail about their difference.

https://www.cybrosys.com/blog/what-is-the-difference-between-extend-and-include-in-odoo-16-javascript

Hope it helps