PARS (Prospect Aggregate Rating System) explained: PARS ranks players using a custom formula created by Rick Mahieu (@PARSlist on Twitter).
Players are graded in 7 categories, each weighted differently. Final scores are between 0-100. Some categories: BB%, K%, wRC+, wOBA, ISO (hitters); xFIP, WHIP, & HR/K/BB rates (pitchers). Not all count equally towards the final score. Other factors: BABIP & LD%/ LOB% and GB%, ML proximity, age-to-level, hit tool/pitcher arsenal, util-/4th OFer-/reliever-risk, track record. Pedigree is factored in as well. A struggling 1st rd pick in A+ gets a higher score than an old-for-the-level teammate drafted in the 28th round w/the exact same stats. He also uses a custom stat for hitters called BBPG (Bombs + Bags per game) to recognize power/speed guys.