Bare Rubber refers to a class of tires. These tires must not have any traction modifiers in them. Just the bare rubber on the ice. Street division drivers run tires that are DOT approved for use on public roads, which are typically studless winter tires, but could also be all-seasons or other daily driving tire compounds. Pro Street and Competition Divisions are allowed to run tires that are designed for ice driving or tires that have a very soft rubber compound. You can check the rule book for more specifications on these classes.