Hi Danny,

> what happens when you split the top surface in half to
> produce a seam down the center?

A "trimming loop" is a topological entity that belongs to a face, and this method is basically a shortcut to help with selecting a trimming loop.

So it is oriented towards selecting edges that belong to one particular face.

If you split the top surface into 2 faces, then you will get a loop that goes around one of them like this:





- Michael