Spinor[p, m, optarg] is the head of Dirac spinors. Which of the spinors u, v, or is understood, depends on the sign of the momentum (p) argument and the relative position of DiracSlash[p]: Spinor[sign p, mass] is that spinor which yields: sign*mass*Spinor[p, mass] if the Dirac equation is applied (by DiracEquation or DiracSimplify).
See also: DiracSimplify, SpinorU, SpinorV, SpinorUBar, SpinorVBar.
The FeynCalc Book | SPD | SpinorU |