This column appears if the setting Use operation overflow? has been configured under the heading Preparation that you will find under the Manufacturing tab in the Settings procedure. Then you can use overflow in the operation list when preparing a part.
You can then enter a fixed quantity for this overflow here. The setup quantity can e.g. be applied when a number of "test run samples" or "setup rejects" have to be run in order to setup a machine for production. This can be used as an alternative to, or in combination with, Extra(%) on the operation row.