It all depends on the individual and what a buyer so to speak would want. For example A snake can be a Val Verde county locality (although this is not common practice) if it a cross between a langtry and a 277. The snake could be just a 277 if it a cross between a Red Bluff and a Mail Trail, or the snake can be a gas station if it is from parents caught right at Loma Alta.
Generally Alterna enthusiasts would want to stick to lines of localities which are identified a being close enough for the original wild caught parents to have been able to have bred together in the wild. Some people believe this is a few miles and some people believe they need to have been found within a few feet of each other, and some people believe they need to be found in the same place and the same season. Whatever it is that you feel comfortable with is really the key. A locality animal to some people is a generic to others, but that is the fun of it, it can be very specific or it can be somewhat general when it comes to locality graybands.