Breaking: models.physics.particles.fields takes a source, as any target does: a name under sources:, {expression, constants}, or {sum: [...]}. A bare number or expression, params and value, and xn, yn, zn are refused with the replacement named.
Particles (experimental): a field's expression also reads xi, eta, zeta, L_xi, L_eta, L_zeta, and the case's constants, rho and nu.
Sources: {background, regions} is part of every source's expression, so a velocity may use it too.