Hi,

There is nothing complex in it, i f you want to add a selection field in a tree view, let it be the sale order line.  If it is sale order line you can check the field Invoice status  field in the sale order line which is invisible, it is a selection field.

Just go to the code and see how it is done,

python:

invoice_status = fields.Selection([    ('upselling', 'Upselling Opportunity'),    ('invoiced', 'Fully Invoiced'), 
   ('to invoice', 'To Invoice'),    ('no', 'Nothing to Invoice')    ],
     string='Invoice Status', compute='_compute_invoice_status', store=True,
         readonly=True, default='no')

XML,

<field name="invoice_status" invisible="1"/>

Thanks