Surface Elo is an Elo rating kept separately for each court surface: hard, clay, and grass. A single overall rating hides the fact that the same player can be elite on clay and ordinary on grass, or vice versa.
Why one rating isn't enough
Surface changes everything: bounce height, court speed, how much a big serve or heavy topspin pays off, and how long rallies run. A grinder who thrives in long clay exchanges may struggle on fast grass where points end in two or three shots. If you only track one number, you systematically misprice every match played away from a player's best surface.
How Smashrs uses it
We maintain a surface-specific rating for every player and use the rating for the surface a given match is actually played on. That makes the model sharpest exactly where one-number systems are weakest: clay specialists in the spring swing, grass-court flukes in the short grass season, and hard-court regulars the rest of the year.
Surface tendencies also feed into a player's archetype, and the resulting probability drives the model edge against the market.