We will create a new interface, ILInkedColumnActivty(IActivity) that will have the value of the term|worksheet|activity triplet as its only attribute. The New Linked Column action button, as well as the edit link in the manage activities view, will call up a table of terms, worksheets, and activities, the activities being links to click on to select the source of the column data.
The only possible activities that are selectable from this view will be activities that come from regular worksheets.
The implementation of the class should work as ILinkedActivity does in that it dreives from IActivity and calls The __init__ method of Activity to fulfill the interface contract.
The view should make the user choose a category, The default will do when the user doesn't want to choose.
In particular, "linked" or "imported" columns like a spreadsheet uses.