Models provide useful insights in the partonic structure of hadrons. However, one has to use great care in the interpretation of the results when the models induce non-trivial relations between parton distributions that are independent functions in QCD.
One example is the model-dependent relation between transverse distortions in the distribution of quarks in impact-parameter space and analogous distortions in transverse-momentum space due T-odd effects.
We discuss the origin of such relation, pointing out the very specific conditions under which it is realised. These conditions are typically verified only in relatively simple models that describe hadrons as two-body bound systems and involve a helicity-conserving coupling between the gauge boson and the spectator system.