stock_inventory_product_categ/views/stock_inventory.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
	<data>

        <record id="view_inventory_form" model="ir.ui.view">
            <field name="name">stock.inventory.form</field>
            <field name="model">stock.inventory</field>
            <field name="inherit_id" ref="stock.view_inventory_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='lot_id']" position="after">
                    <field name="categ_id"
                           attrs="{'invisible': [('filter', '!=', 'category')],'required': [('filter', '=', 'category')]}"
                           />
                </xpath>
            </field>
        </record>

    </data>
</openerp>