Paint shop pro has three "selection" tools.
http://mtm2.com/~mtmg/trucks/painting1.htmlYou use those to indicate what part of the image you are editing. A selection marks off an area so that nothing outside of it is effected. Now, when you select, the edges are usually razor sharp and a fixed size. We can change that by expanding or contracting the selection area. And we can soften the edge so that it blends from one paint surface to the next.
Try an experiment. Create an image 400x400 pixels. Now select an area in the middle and fill it with a different color. Notice how hard the edges are - like they're drawn with a ruler. Now do the same thing again but this time feather the selection 10 to 20 pixels before filling - the result is a gradient blend instead of a sharply contrasting line.