It is described in the section 5.1.4.1 «3D_XID» (page 31) of the «Direct Post Integration Guide», version 1.4, 2017-02-28:
| CLASS | Mandatory when EPS_TXNTYPE includes 3D Secure |
| FORMAT | String, length 20 |
| DESCRIPTION | 3D Secure Transaction ID string. MUST uniquely reference this transaction to the Merchant, and MUST be 20 characters in length. Any ASCII characters may be used to build this string. E.g. May comprise of a timestamp padded with 0s for uniqueness: `20110714112034872000`. |
| TYPICAL USE | `` |
