What does "same industrial group" mean?

Operations at a site that belong to the same three-digit North American Industry Classification System (NAICS) code prefix (which has replaced the old SIC codes) belong to the “same industrial group. In addition, where one or more operations at the site serve primarily as support facilities for the main operation at the site, the supporting operations are part of the “same industrial group” as the main operation. For example, if you manufacture chemicals (NAICS 325) and operate a waste treatment facility (NAICS 562) that handles primarily wastes generated by your chemical operations, the waste operation would be considered a support operation. If you operate a petrochemical manufacturing operation (NAICS 32511) next to your petroleum refinery (NAICS 32411), the two plants would be considered in different industrial groups and would require two RMPs unless the majority of the refinery’s production was used by the chemical manufacturing plant.