purchase_global_discount/security/purchase_security.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data noupdate="0">
    
    <record id="group_discount_per_po_line" model="res.groups">
        <field name="name">Discount on purchase lines</field>
        <field name="category_id" ref="base.module_category_hidden"/>
    </record>

</data>
</openerp>