This terminology seems to be referred to a notion invented in an old book of Oliver Heaviside . What is it and why the Biot-Savart law is not unique unless it is applied to a Heaviside rational current element as claimed in this paper ? Moreover, why such direct-current element is a physical impossibility?
 Oliver, Heaviside. Electrical Papers, Vol.2, Article 48. 1892  Hammond, P. "Electric and magnetic images." Proceedings of the IEE-Part C: Monographs 107, no. 12 (1960): 306-313.