How to divide points into two sets - upper and lower part of profile

How to divide points into two sets - upper and lower part of profile

I have lists of points in 3d (x,y,z) For each list I want to divide that
list into to two lists, one containing points from upper part of profile
and second the lower half, just like that:

My question here is how to determine which point should go to upper-part
and which one should go to lower part just from having those points with
their coordinates (x,y,z).