is a fundamental question of QCD which we want to answer. To answer this question unambiguously, we cannot avoid to clarify the following issues. What is a precise definition of each term of the decomposition based on quantum chromodynamics (QCD) ? How can we extract individual term by means of direct measurements? Let us call it the nucleon spin decomposition problem [@JM90]-[@BLT04].

The recent papers by Chen et al. [@Chen08],[@Chen09] arose much controversy on the feasibility as well as the observability of the complete decomposition of the nucleon spin [@Tiwari08] -[@Lorce12]. In the previous papers [@Wakamatsu10],[@Wakamatsu11A], we have established the existence of two physically nonequivalent decompositions of the nucleon spin, both of which are gauge-invariant. The quark and gluon intrinsic spin parts of these two decompositions are nothing different in these two decompositions. The difference appears in the orbital parts. The quark and gluon orbital angular momenta appearing in one decomposition is gauge-invariant [*dynamical*]{} (or [*mechanical*]{}) orbital angular momentum (OAM), while those appearing in another decomposition is generalized [*canonical*]{} OAM having gauge-invariance. The key quantity, which characterizes the difference between these two types of OAMs, is what-we-call the [*potential angular momentum*]{} [@Wakamatsu10]. Understanding its physical meaning is therefore of vital 