x1 (vector) is the displacement of x2 (object/point) relative to x3 (object/point; contextless default: origin) in coordinate system/frame of reference/as measured by x4 according to definition/standard/(meta)physics x5
x1 is (four-)vector-valued; it is a function that maps to a tensor (probably a vector or scalar) to a (four-)vector. Units are handled in x1, which will be a (four-)vector of mitre or the like. Time of measurement is just «ca» or the argument of the function(s) in x1, or can be defined instantaneously by x4. Proposed short rafsi: -nif-. See also: nifkemtemsalri.