Ram: Here's one way to look at the words on page 251.
Example: you planned a project to be three months long. At the 45 day mark you only 25% complete.
So Planned vs Actual (the actual is your WPI). 50% vs 25%
The difference between both numbers is -25%. This amount is "data". You take this data and do things like make a forecast for Estimate at Completion (see page 220)
The next step for you, as the project manager, is to translate this "data" into something people can act upon. Find out what is going on and report it to your sponsor, along with what you need to fix it. Maybe you need more time, money, or resources.
Good question, by the way. WPD, WPI, WPR seems to cause many people confusion, but when you sit down and study it, it becomes quite clear.