Hello
The NEC stands for the National Electrical Code and is published and owned by the NFPA.
Basically, the NEC is used by electrical installation personnel for rules of installations of equipment. For example, "NEC article 500" is used for hazardous location installations.
The NEC is for installation of equipment and ATEX is for manufacturing of hazardous equipment. Sort of like UL would approve equipment under some standards such as UL1203 or 913.
To say that ATEX equipment is superior would be very difficult to prove. For example an Ex d enclosure may only have 5 explosion tests done inside it where the UL Class I Div 1 piece would have had a minimum of 10 explosions. Flame paths and other tests are also different.
The major difference between an ATEX (Zone Classification only) approved piece of equipment and something approved for a Class I Div 1 piece is that the ATEX piece has to comply with all other directives that may apply to it such as EMC/EMI, low voltage, and the manufacture has a QA or ISO system in place, and has be tested by a Notified Body in Europe.
There are many things I have not mentioned here and of course there are execeptions to the rules.
Your customer may have meant that he/she wants the equipment to be installable as per the National electrical code (most likely Class/Div but possible Zone) and bear markings of an approval agency such as UL/CSA/Entela.
Because natural gas is involved, the area the equipment is going into would be a classified area so the equipment should be marked say Class I (gas vapours) , Div 1 or 2 ( whichever applies) group D.
If it was for Zone, then Ex "and then whatever method of protection is used" and IIA plus the T code.
After reading over what I wrote, I realize that I jumped around a lot so I hope it makes sense.