Its the amount the index into the existing content list should be incremented after the items have been removed.

In common operation this is 0 since the items have been removed. But there's another way of syncing where instead of replacing the content of the old list with the new list both lists are live at the same time and a couple of offset pointers are used to reference the correct one.  In that case the items aren't removed so the correct return value would be the count. There are other scary use cases as well.