Archive number has been added to the invoice module
(available from V6.4.826)

TO -> Orders

The field "archive number" has been added. This field will be filled during the save process only if you have done the following settings on the voucher type:

Basic data -> Company index -> Voucher types

You have to set the check for "archive entry" under the options. On the tabpage "Invoice" there will be displayed an additional column "archive number".