The report on plans and priorities was designed to support the study of the main estimates. So when you actually look at the main estimates, the theory behind the report on plans and priorities is that it's a document, by department, to help committees complete their studies of the estimates. So it gives additional details for each department to help in the study of the estimates.
The report on plans and priorities was not designed to help assess the budget. The link for the RPP is the main estimates.