  High energy hadronic scattering processes can be described in terms of
partonic scattering processes and parton distribution and fragmentation
functions. These are bilocal matrix elements of quark operators. Colour
gauge-invariance requires inclusion of a gauge link, which is characterized by
its path. For transverse momentum dependent distribution and fragmentation
functions, appearing in single spin asymmetries, different paths appear
depending on the process. The difference is related to a time-reversal odd
gluonic pole matrix element. We discuss briefly the path structure of the gauge
link in more complicated processes.
