Non-roster invitee

From MLB4Uwiki

Non-roster players are players signed to minor-league contracts who attend major-league spring training on invitation. These players can be added to the opening day roster by having their contracts purchased and being added to the 40-man roster. A non-roster player with at least five full years of major league experience has the right to refuse assignment to the minor leagues during the first 45 days of the season unless he signs a waiver stating that he cannot choose this option.