A click-through is an instance on the last-touch channel. It is an eVar with the most recent allocation.
For example, assume that a visitor comes to your website once a day, with each visit originating from a different marketing channel:
The First-Touch Channel report would show 1 new engagement for Paid Search. Each other channel would show 0 new engagements. The Last-Touch Channel report would show 2 click-throughs for Paid Search; 3 for Display; and 2 for Natural Search.