SpinorU[p, m] denotes a u(p,m)-spinor that depends on the 4-dimensional momentum p.
Overview, Spinor, SpinorUBar, SpinorV, SpinorVBar, SpinorUBarD, SpinorUD, SpinorVD, SpinorVBarD.
SpinorU[p, m]u(p,m)
SpinorU[p, m] // FCI // StandardForm
(*Spinor[Momentum[p], m, 1]*)SpinorU[p]u(p)
SpinorU[p] // FCI // StandardForm
(*Spinor[Momentum[p], 0, 1]*)GS[p] . SpinorU[p]
DiracEquation[%]\left(\bar{\gamma }\cdot \overline{p}\right).u(p)
0