Here is the applicable code on subject:
(K) Combustible Gas Detection System. A combustible
gas detection system shall be permitted as a means of protection in industrial establishments with restricted public access and where the conditions of maintenance and supervision ensure that only qualified persons service the installation.
Gas detection equipment shall be listed for
detection of the specific
gas or vapor to be encountered. Where such a system is installed, equipment specified in 500.7(K)(1), (K)(2), or (K)(3) shall be permitted.
The type of
detection equipment, its listing, installation location(s), alarm and shutdown criteria, and calibration frequency shall be documented when combustible
gas detectors are used as a protection technique.
FPN No. 1: For further information, see ANSI/ISA-12.13.01,
Performance Requirements, Combustible Gas Detectors.
FPN No. 2: For further information, see ANSI/API RP 500,
Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division I or Division 2.
FPN No. 3: For further information, see ISA-RP12.13.02,
Installation, Operation, and Maintenance of Combustible Gas Detection Instruments.
(1) Inadequate Ventilation. In a Class I, Division 1 location that is so classified due to inadequate ventilation, electrical equipment suitable for Class I, Division 2 locations shall be permitted.
(2) Interior of a Building. In a building located in, or with an opening into, a Class I, Division 2 location where the interior does not contain a source of flammable
gas or vapor, electrical equipment for unclassified locations shall be permitted.
(3) Interior of a Control Panel. In the interior of a control panel containing instrumentation utilizing or measuring flammable liquids,
gases, or vapors, electrical equipment suitable for Class I, Division 2 locations shall be permitted.
(L) Other Protection Techniques. Other protection techniques used in equipment identified for use in hazardous (classified) locations.
It has only 3 areas of use listed. If any of the 3 apply you are good to go. As far as 2nd part of question, I would hope if you refer to info referred to in FPNs, you would get your answer.