SpinorUD[p, m] denotes a u(p,m)-spinor that depends on the D-dimensional momentum p.
Overview, Spinor, SpinorUBar, SpinorU, SpinorV, SpinorVBar, SpinorUBarD, SpinorVD, SpinorVBarD.
SpinorUD[p, m]u(p,m)
SpinorUD[p, m] // FCI // StandardForm
(*Spinor[Momentum[p, D], m, 1]*)SpinorUD[p]u(p)
SpinorUD[p] // FCI // StandardForm
(*Spinor[Momentum[p, D], 0, 1]*)GSD[p] . SpinorUD[p]
DiracEquation[%](\gamma \cdot p).u(p)
0