You can save a rule for re-use when you set up a new rule on a device.
You might wish to do this if you want to add the same rule to another device.
All you need to do is tick this box when setting up a new rule:
If you ticked this box, when setting up the rule then you will see it in the list below your rule templates. You can then select it and apply to other devices without having to re-create it.