Complex sentence: The other sort of variables, called "set variables" or "class variables" or "predicates" and usually written in upper case, refer to classes/predicates/properties of individuals, which can be thought of as sets of natural numbers.
Split into simpler sentences: The other sort of variables, variously called "set variables," "class variables," or even "predicates" are usually denoted by upper case letters. They refer to classes/predicates/properties of individuals, and so can be thought of as sets of natural numbers.