I've researched the ATPL manuals I have and it basically says that in the atmosphere we have "compartments" of areas of convergence and divergence, which I would take to be as low and high pressure systems, and that between them you have a level where divergence/convergence is zero, which occurs at the 500 hpa pressure surface (FL180). Now when I look at current surface weather charts and upper level weather charts, I do not see this theory confirmed.
I think the theory is overtly simplistic to enable examiners to set solvable questions. The basic theory as I understand it is that the wind will follow lines of constant pressure (isosomthingelses) at mid to upper levels. Same law of NH wind on back with low pressure to left. So upper level convergence and divergence is really a bit of a red herring insofar that all upper level pressures are relative and a surface low does not necessarily translate to an upper high if the temperature gradients and relative heights of the various columns are taken into account.