Hi,

Either you have to group the products manually in the order lines based on the product category or it has to be done using by the code by giving some sequence for the product categories. Hope you have asked the same before:  How can I make a purchase order ordered by product category?

An workaround that you can try if you didn't succeed with the above is, create a new menu called Purchase Order line and define the search view for that view. Now from here you can easily filter and group by the order lines as per the need.

For defining new menu and action : Define New Menu and its Actions in Odoo

Thanks