N.J. Stat. § 19:4-10
As nearly as practicable, each election district shall be composed of contiguous and compact areas having clearly definable boundaries and shall be contained wholly within only one ward, only one municipality, only one county freeholder district, one State legislative district, and only one other district from which any public official is elected, except that for the purpose of establishing Congressional districts pursuant to Article II, Section II of the New Jersey Constitution, an election district may be split between two such districts. The election districts within each municipality shall be numbered consecutively. If any election district is split between two Congressional districts, one of the districts shall also be assigned the letter "a" and the other district shall be assigned the letter "b".
N.J.S. § 19:4-10