The Assistant Majority and Minority Leaders of the Senate of Puerto Rico (commonly called Majority Whip and Minority Whip) are two Puerto Rican Senators who are elected by the party conferences that hold the majority and the minority respectively. These leaders substitute the Majority and Minority leaders in their absence.
The "Majority" column indicates which party was the majority in the Senate, while the opposing column indicates the minority. The PIP is usually a minority with just one senator, which also precludes it from having an assistant leader.