40 CFR 51.166(b)(5) defines "stationary source" as any building, structure, facility, or installation which emits or may emit a regulated NSR pollutant.
40 CFR 51.166(b)(6) further defines "building, structure, facility, or installation" as all of the pollutant emitting activities which belong to the same industrial grouping, are located on one or more contiguous or adjacent properties, and are under the control of the same person (or persons under common control) except the activities of any vessel. Pollutant-emitting activities shall be considered as part of the same industrial grouping if they belong to the same Major Group (i.e., which have the same two-digit code) as described in the Standard Industrial Classification Manual, 1972, as amended by the 1977 Supplement (U.S. Government Printing Office stock numbers 410)."
In summary, there are three distinct and independent criteria that must be satisfied in order to be considered part of a single stationary source; 1) common legal control; 2) contiguous or adjacent properties; and 3) part of the same 2-digit SIC Code.
The concept of support facility was introduced by EPA in the 1980 preamble to the PSD regulation to elucidate the SIC code criteria. Using the support facility concept, an emissions unit that conveys, stores, or otherwise assists in the production of a primary product may be considered part of the stationary source's primary activity. See 45 Fed. Reg. 52695. However, the use of the support facility concept is not related to the other two stationary source criteria.
NCDAQ Stationary Source Determination, SEFA (July 3, 2008)