The final step is used to initiate the creation of the prepopulated pages ready for your budget holders to review and edit. When you press the Save button PMI will create a set of plan pages for the period specified with the name you set and adjust the titles to refer to it as budget or forecast or target as specified.
It will take a copy (snapshot) of values in the Live forecast at that point and use them to display the usage/consumption values in the plan pages. It will also take the set of comparative data specified and display it in the comparative data section of the plan pages.
It will also use the comparative data and any other settings like the inflation % to calculate the unit prices to use in the price section. The new plan pages will then be prepopulated with a starting point for all cost items and months that have historic data and a forecast activity level in Live forecast.