A motion to Postpone Indefinitely is used to:

A motion to Postpone Indefinitely serves the purpose of effectively ending consideration of a main motion without directly voting it down. When a member makes this motion, they are essentially suggesting that the assembly should not take any further action on the main motion at hand. If the motion to Postpone Indefinitely passes, it acts as a way to "kill" the main motion, ensuring it cannot be brought back for discussion in the same meeting. This is particularly useful when members want to express their opposition without going into a potentially lengthy debate about the main motion itself.

The other options do not accurately reflect the purpose of a motion to Postpone Indefinitely. Delay a decision implies merely putting off a discussion without concrete action, while modifying a motion suggests altering its terms, which the motion does not do. Referring the motion to a committee involves sending it for further consideration elsewhere and does not terminate it. Thus, choosing to Postpone Indefinitely clearly aligns with the action of killing the main motion.