What P-1 means is that it's a lot easier to install this gasket from the outside of the tub. Usually there is a plumbing access panel, from which you can access the tub drain. If not, you'll have to scrunch up the gasket to get it through the overflow hole from the inside.
I did it from the front and it was fine. Not that hard. Thank you Verdeboy. I also bought some product at Pool City to clean the jets, and that worked fine, too.