Replenishment is a manual process.  Users go to the Replenishment view and can take action on any products that need to be ordered.

Reordering Rules are automatic.  There's a daily scheduled job that checks what needs to be ordered, and creates Purchase Orders, Manufacturing Orders, etc.

One database table stores both:

• automatic reordering rules and
• manual reordering rules (used for Replenishment)

and Odoo processes both of them in a similar way, which make it easy to switch between them.

https://odootricks.tips/odoo-14-replenishment-mto/