Complex sentence: PTOL is assisted in certain cases by means which are not needed by VTOL or by STOL, e.g. a very long cable enforcing the convergence of the aircraft from afar towards the intended touchdown point.
Split into simpler sentences: PTOL is assisted in certain cases by means which are not needed by vertical take-off and landing (VTOL) or by short take-off and landing (STOL) vehicles. These include a very long cable enforcing the convergence of the aircraft from afar and the intended touchdown point.