Solved
Gathering part and the related section in Help Center
In Help Center there's a topic "Filtering Gathering Parts" which in the end says:
"
To assign a gathering part attribute to a Windchill part:
1.Set the value of the server-side preference, Operation > Auto Associate > Phantom Assembly Override Attribute Name to a BOOLEAN customer-defined soft attribute
2.In the CAD application, set the attribute to yes
Auto Associate creates a Gathering Part attribute on the Windchill part when it is created.
"
Does this mean:
- create a soft attribute to EPMDocument
- enter this as value to the mentioned preference
- in CAD application use this to define whether the associated WTPart should be a Gathering Part or not
- when the EPMDoc is checked in the related WTPart's Gathering Part attribute is updated accordingly
This being a "customer-defined soft attribute" on the EPMDoc changing the value iterates the EPMDoc?
While Gathering Part -attribute on WTPart is a common attribute and changing it does not iterate the WTPart?

