This field contains an evaluated formula where the result is used to load the 'category' heading in the attachment, when an attachment is created on archiving the document. This is a numeric value defined by the local menu 96.
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.