What does $\alpha_r\supset\beta_r$ mean in the tolerance definition of System Z?
In elementary logic, $A \supset B$ is notation for the formula $\neg A \lor B$ (for the implication "$A$ implies $B$").  In other words, $A \supset B$ is a formula; it is true if $A$ is false or if $B$ is true, and false otherwise.  https://math.stackexchange.com/q/1106001/14578

How you could have figured this out on your own: Read the entire paper.  For instance, a useful place would be to start would be reading the examples in Section 3, where from the examples one can reverse-engineer the probable meaning of the symbol.