old_contents: # License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl).
  from odoo import models, fields
  
  
  class AccountTax(models.Model):
      _inherit = 'account.tax'
  
      payability = fields.Selection([
          ('I', 'Immediate payability'),
          ('D', 'Deferred payability'),
          ('S', 'Split payment'),
      ], string="VAT payability")
  
new_contents: # License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl).
  from odoo import models, fields
  
  
  class AccountTax(models.Model):
      _inherit = 'account.tax'
  
      payability = fields.Selection([
          ('I', 'VAT payable immediately'),
          ('D', 'unrealized VAT'),
          ('S', 'split payments'),
      ], string="VAT payability")
  
subject: Use correct english terms, from APPENDIX A -TECHNICAL SPECIFICATIONS