Indicates the e-commerce index.

Its value is populated by 3DS authentication server (ACS) when the buyer has been correctly identified (vads_threeds_status equals "Y" or "A").
  status =Y status = A status = U status =N
VISA and AMEX 5 6 7 -
MasterCard 02 01 - -

Input and output field, returned in the response (IPN and Return URL).

Format n..2
Error code 53
Category 3DS Authentication.