x1 is the vector flux (flow) of quantity/substance (count) x2 through (geometric/imaginary) hypersurface/embedded manifold x3 per unit hyperarea per unit time.
This is the transport phenomena sense of "flux": a density of flow, the maximal rate of flow of a subsramce or quantity (x2) through a differential hypersurface. If the hypersurface x3 is n-dimensional spatially and the dimensionality (units) of x2 is U, then the dimensionality (units) of x1 is U/(1 mn * s). Fairly similar to nildenmi. For the hypersurface integral of this quantity (with respect to the normal), use: flaumji. See also: flecu.