What is the CERCLA petroleum exclusion?

The term "hazardous substance" is defined in CERCLA section 101(14) to include substances listed under four other environmental statutes (as well as those designated under CERCLA section 102(a)). The definition excludes "petroleum, including crude oil or any fraction thereof," unless specifically listed or designated under CERCLA. Additional clarification on which substances are excluded by the petroleum exclusion is provided in reference here.