Just to expand on what
@Mdragger88 said above.
Overlap liners tend to not have any sort of waterline pattern, kind of like fake waterline tiles. With a Unibead or J Hook the waterline pattern is pretty much guaranteed to be level (as long as the to of your wall is level). An overlap has a lot more play in it. Waterline patterns in overlaps are next to impossible to get level, and end up looking horrible when filled with level water. That is why you usually only find overall random pattens on overlap liners.