In the scheduling section of the rule creation screen (find this at the bottom of the page), if you choose 'Reactive' then the polling mechanism or event-driven mechanism is used. Alternatively, if you want the rule you're creating to only process orders at specific times of day, you can choose a 'Daily', 'Weekly' or 'Monthly' schedule.
You can also choose a one hour slot in which your orders will be processed but be aware that if you have a large number of open orders, it may take more than one hour to process all qualifying orders in which case the time slot will overrun.
The schedule will consider all orders which match the conditions oldest to newest, so is a great approach if you want to allocate or fulfil your oldest orders before your newest when new stock is made available.
Choose daily if you want a rule to be run at a specific time of day, every day of the year. Once you select this option, you'll then be able to the time of day the rule will run.
Choose weekly if you want the rule to run only on days of the week that you choose. Once you choose the active days, select the time of day the rule will run.
Choose monthly if you want the rule to run only on a specified day or days of the month. Once you select this option, you'll then be able to the time of day the rule will run.
Should you choose a day that doesn't occur in the current month (for example, setting the 30th of every month, and it's February), the rule will be run in the last available day (February 28th).