[SecurePay] What is «3D_XID» in a 3D Secure authentication scenario?

It is described in the section «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`.