In the couple of days I started reading Excelisys' drag and drop solution (from Andy Persons) and found something strange in there script namely the _trigger = Self. https://www.excelisys.com/fm-tips/drag-and-drop-using-the-separation-model/
Let ( [
_trigger = Self;
$$move_id = category
This script is essential while we move the content of a container onto another container it picks up the targeted container's category, change the original category to a new category. However I am not sure how it does it perhaps the "_trigger = Self; " responsible for this but "_trigger" as a variable does not refer to any calculation it is just stand alone. They explain that it ensures that the calculation will run any time the field is modified. I think there is something in filemaker undocumented.