The NOM or Norma Oficial Mexicana that you see on tequila bottles are a four digit number that is assigned to the distillery which was responsible for producing the brand of tequila. More advanced tequila sippers make decisions on purchasing tequila that comes from specific distilleries as they usually share the same process. Using websites like Tequila Matchmaker,You can look up specific products and brands that were made by entering a NOM found on the bottle. This is also how most people find other brands to try made in a distillery that they favor.