Automatic Scoring is the default that gives each Score Band an equal number of points. That means with 100 points and three Score Bands, Automatic Scoring makes each Score Band the same size: Low from 0 to 33, Medium from 34 to 67, and High 68 to 100.
With Manual Scoring, you control the size of each Score Band, so you can make some larger or smaller. For example, with 100 points you could make Low from 0 to 25, Medium from 26 to 74, and High from 75 to 100.