The purpose of the Plan tab is to allow you to define your intended experimental design separately from, and in advance of, the actual execution of the experiment. I.e., it’s like writing down your experimental design in a notebook before you go into the lab to do it. Separating this data into Plan allows you to see clearly if you (or some colleague) deviates from what was intended.
It’s not always necessary put the planned values in plan. You might just put that plan data in the Actuals tab before your experiment, and then change the Actuals data if you deviate. This is OK too, and may be easier, but you will lose the opportunity to track the deviations between intended values (Plan) and the values actually observed (Actuals).
Note that if you put target values on your properties in the process design, the will be displayed as the Plan for that property for all runs, unless you override that default target value by adding data in the Plan tab.
We also note that there is currently a bug in Plan mode that prevents you from performing fill-down operations. And you can’t use the run grouping (aggregation) features in Plan. These issues make Plan tab a bit harder to use, particularly for large run counts. We are planning to fix these issues by merging Plan mode more seamlessly into Actuals.