RSF or Rubber Semi Flat, is an SF Bottom completely made out of rubber.
It is essentially a flat tip with a smaller radius, affording it a firm, defensive grip on the Stadium, but still allowing for a somewhat aggressive movement pattern with the proper launch.
Its drawbacks are that it has shown major weaknesses against Left-Spin Beyblades such as Lightning L Drago, much like RS. RSF, like most Rubber Performance Tips, suffers from poor Stamina, although it has been shown to have slightly more Stamina than RS, due to a wider surface area, resulting in better balance at the end of a battle.
Compared to RDF and even RF however, RSF is not the preferred choice of tip even in Defense, suffering from the eternal issue Balance parts have of not excelling in any type and instead performing decently in two or more categories.
IMPORTANT: Mold Variations
RSF has two Mold variations. The RSF's included in Random Booster Vol. 7 Beat Lynx, specifically with the Bey Cyclone Horuseus 130RSF. This mold appears to be made of a softer rubber, and thereby behaves more aggressively than other molds. This should be taken into consideration when choosing a method of obtaining an RSF. The RSF included with Blitz Unicorno 100RSF is of the harder rubber variant.