Note that every Action has a recommended range indicated in front of the priority input field. If you really know what you are doing, values outside of the recommended range can be used.
Within each Runtype, order of execution is determined by
In other words: Actions with the same priority are executed by title alphabetically (unless brought forward within the same Runtype through references or dependencies).
This way you can easily insert Actions later on using 1,500 or similar priorities.