“Line objects” means constellation lines and boundary lines. PP3 treats both of them in a special way: When they collide with the rim of a label, it's less bad than for non-line objects. The reason is that lines are so different from labels visually, that it's not fatal if they are rather close together.
The two new penalties that are used for the rim are
boundaries_rim. Their default
1000 as usual. For example, with
penalties boundaries_rim 0
you tell PP3 that a label directly next to a boundary line is totally okay for you.