An unsigned player under club control who does not yet qualify for salary arbitration ultimately has to either accept the club's offer or just not play.
A club will negotiate with the player up to a point, but if the player has not signed a contract for the current season by March 1st, the club has the right to unilaterally dictate the player's salary and renew the player's contract from the previous season (albeit for an amount not less than the MLB minimum salary, and not less than 80% of the player's salary from the previous season and not less than 70% of the player's salary from two season's back).
These players are the ones who have a "minor league split" salary in their contract, which the player is paid if he is sent to the minors. A player's "minor league split" salary must be at least equal to the 2019 MLB "minor league split" minimum salary and must be at least 50% of the player's salary from the previous season.
NOTE: If a free-agent signs a major league contract with a minor league "split" salary, the "50% rule" does not apply.