25 Pa. Stat. § 3291
The several classes of nominations at primaries and elections of public officers which may be contested in this Commonwealth are hereby distinguished and designated as follows, to wit:
Class I. Nominations and elections of the Governor and Lieutenant Governor of the Commonwealth.
Class II. Nominations and elections of electors of President and Vice-President of the United States and all officers of this Commonwealth, including Judges of the Courts (except Governor and Lieutenant Governor), who now are or hereafter shall be required to be nominated or elected by the electors of the State at large, and nominations of United States Senators.
Class III. Nominations and elections of judges of the several courts.
Class IV. Nominations and elections of Senators and Representatives in the General Assembly, and nominations of Representatives in Congress.
Class V. All other officers, whether nominated or elected by the qualified voters of counties, cities, boroughs, townships, wards, school districts, poor districts or any other division of the State.
25 P.S. § 3291