Thank you for your answer Milind, very helpfull. I have an additional question:
Does this also mean that when I make a keyword volume search for a given location "A" (a city or a zip code for example) through the "Keyword planner" tool the index I get is the search volume for the given keyword within the admistrative area of location "A"?
In other words, is the geolocation accurate enough so that we are sure that the index is representative of the population living exactly within the administrative boundaries of location "A"?
Thanks,
Antoine