It is a fancy name for a set of data you are comparing your draft plan against.  The dataset shown under comparative data is also used to calculate other elements of your new plan.  Look for ‘based on comparative data’ in a section header like this: 

Comparative data is often just last year’s actual costs for the department and cost type we are looking at.  We have given it a general title because the admin user doing setup can choose what the plan will be compared to, and can also mix and match several sources as in this example.

When doing the setup, the admin user can choose between actual data from any past year that is in PMI and any of the previous forecasts/budgets created through the planning module.  As a forecast file can be longer than one year they can also use multiple sources to cover all the months.