Field help (ARCIDTCNT) 

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 reference is the identifier of the container where the document will be placed in the EDM. The container can be a directory, a machine or an URL.

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.