The purpose of this function is to allow users of the i21 system to search by a historic account number (or any other cross reference) when looking up accounts - this should help if customers use another GL system they are consolidating into or if they are converting to i21 - so they know the historic reference to look up.
Configure the system to use that map in lookup.
Results:
When you have configured the system appropriately you will note the alternate account id will display in the account lookup window