  In a series of papers, we have established the existence of two
gauge-invariant decompositions of the nucleon spin, which are physically
nonequivalent. The orbital angular momenta of quarks and gluons appearing in
these two decompositions are gauge-invariant dynamical orbital angular momenta
and "generalized" canonical orbital angular momenta with gauge-invariance,
respectively. The key quantity, which characterizes the difference between
these two types of orbital angular momenta is what-we-call the {\it potential
angular momentum}. We argue that the physical meaning of the potential angular
momentum in the nucleon can be made more transparent, by investigating a
related but much simpler example from electrodynamics. We also make clear
several remaining issues in the spin and momentum decomposition problem of the
nucleon. We clarify the relationship between the evolution equations of orbital
angular momenta corresponding to the two different decompositions above. We
also try to answer the question whether the two different decompositions of the
nucleon momentum really lead to different evolution equations, thereby
predicting conflicting asymptotic values for the quark and gluon momentum
fractions in the nucleon.
