"Select Products" button in invoice form view.

When you click "Select Products" button "Select Products" wizard will open.

When you click "Add a line" another add:products wizard will open and select products as you want and press select button.

Now, selected products added in "Select products" wizard and press "apply" button.

Products are automatic added from wizard to invoice lines.

In products tree view, select products as you want and click Action -> Create Invoice.

In Below wizard, select "Invoicing Type" and "customer/vendor" and hit "Create Invoice" button.

Now, Invoice created and selected products added in Invoice lines.

In product variants tree view, select products as you want and click Action -> Create Invoice.

In Below wizard, select "Invoicing Type" and "customer/vendor" and hit "Create Invoice" button.

Now, Invoice created and selected products added in Invoice lines.

You can also add multi products in "Customer Credit Note".

You can also add multi products in "Vendor Bill".

You can also add multi products in "Vendor Credit Note".