  This report attempts to capture the essential workings of gauge links (Wilson
lines) inside gauge-invariant formulations of parton distribution functions in
QCD and gain some deeper insight into their key (renormalization) properties.
We show, in particular, that the one-loop anomalous dimension of the
Cherednikov-Stefanis quark TMD PDF is in the lightcone gauge $A^+=0$, combined
with the Mandelstam--Leibbrandt pole prescription, the same as that obtained in
the special covariant gauge $a=-3$, leaving no uncanceled rapidity
singularities.
