Hi,
You can define the field as a text field and use a default or onchange function based upon your model to assign the value.
The invoice terms is defined on the res.company model , so it is fetched from 'company_id' field if the company_id field is present in the model or from 'self.env.company'.

narration = fields.Text(string='Terms and Conditions')

@api.onchange('field_a')
def _onchange_field_a(self):
    if self.env['ir.config_parameter'].sudo().get_param('account.use_invoice_terms'):
        self.narration = self.company_id.invoice_terms or self.env.company.invoice_terms

Regards