How does a node vote "against" a proposal?

In the SCP white-paper it’s mentioned that when a node votes on a proposal, it promises that it has never voted against that proposal.

However, how exactly would a node vote "against" a proposal? Does voting against a proposal imply just abstaining from voting for that value? There are Prepare, Nominate, Confirm, and Externalize message types, but I am not aware of any "RejectNomination" type of messages.

Edit: Specifically, it’s mentioned that a node may nominate multiple proposals. So what does it mean to reject a proposal?