x1 [signed quantity] is the the intrinsic spin/signed spin quantum number/magnetic quantum number/Dirac-Pauli spin-like charge [commonly denoted: s] of particle/thing x2, as measured along axis/in direction/measuring component x3
Contextless default for x3 is probably along the z-axis in the positive direction or any axis at all. For an electron, this quantity has absolute value of hbar/2 [the Lojban ignores the technical issue in English of "spin" versus "spin quantum number"]. Pay attention to the fact that plankexu is defined as equalling hbar/2, not just hbar. See also: plankexu.