Is it something like the Dewey Decimal System for libraries?
My understanding is that NAICS, like Dewey, has first digits for broad categories, then second digits for subcategories, third digits for narrower categories, etc.
Dewey has six or more digits, but only the first three are whole numbers. (There is a decimal point after the first three). NAICs seems to have five or six digits, all whole numbers.
How do I tell where the NAICS numbers start and stop. And if I see a NAICs number like "511," does it mean something like 511xxx, or more like 000511?