The ID Field is a field which displays the internal GUID of the current record. The GUID is the internal ID used by Sage ACT! to uniquely identify a record. This tool really copies the GUID to a field of your choice: this is useful if you are exporting and reimporting ACT! records in the database as this field can serve as a way to identify records without any duplicate checking.
Updating the records after inserting the tool in the layout
Just after you install the add-on, create a lookup of all records of the database, then right-click the field and choose Update Current Lookup. Once this is done, you don't have to do this ever again. The field is automatically updated each time you create a new record.
See how you can insert it in your layout and how it works.