Field help (ARCIDTSTO) 

This field is used to load the heading that identifies the document in the attachment, when an attachment is created on archiving the document. This document number is the unique identifier of a document in a container of the EDM (knowing that there can be a version number as additional identifier if a single document is submitted several times).

This is a calculation formula using all the variables likely to exist in the context (this depends on the triggering event) and especially the G_XXXX variables where XXXX is the code of a setup passed in an argument in the Variable tab.