This Politico article explains clearly the Pennsylvania presidential primary system, in which voters had to choose delegates to the national convention with no voting cues on the primary ballot. Candidates for delegate run against each other, and the Pennsylvania ballot gives them no label relative to whom they will support at the convention.

A somewhat similar system was once used in presidential general elections in Florida. Florida November ballots before 1931 had no party labels for any partisan office. Furthermore, voters voted directly for candidates for presidential elector, and the ballot didn’t say whom the electors were pledged to. The only voter cue was that the candidates of the Democratic Party were always listed at the top of the ballot.