This option allows you to always have a rule armed once applied to a market, for example if you have a green up rule which greens to a target profit or loss so you might want to use this option to ensure its always armed and always ready to trigger no matter when you choose to open a position.
Here you also have the option to stop a rule being able to trigger after a specified number of seconds from it being started, this acts as a sort of fill/kill feature for a rule and is mainly used when starting rules manually with 'Automation Servants'.
