Bahmni integrates OpenMRS with OpenERP which is a an open source ERP product. It is useful in Billing, Pharmacy and Stock management. Any patient registered in OpenMRS is synced by Bahmni as a customer in OpenERP. 

Since Odoo/OpenERP is an open source product with thousands of existing customers and a community – its own online documentation is also available online here: https://doc.odoo.com/book/#books-link. You can also refer to this documentation to understand OpenERP better, or in cases where you need more clarity than the documentation provided by Bahmni on OpenERP.