I have gotten this issue from underwriters mainly if I have a comp over 5 miles away. Usually I remind them (without telling them to take their head out of their a--- , ummm, rule book) that the reason it is five miles away is because the subject is an oddball and not because of the neighborhood. However, I did get a request once because the town is "in the sticks". I told her that in appraisal terms, there is no set, universally agreed to criteria for rural. However, the conventional wisdom is if it is an unincorporated community and/or more than 30% of the local economy is agricultural, we call it rural. Any other type is suburban. If the property is located in an area of a city that is predominantly commercial or otherwise non-residential, it is urban.
That is just about how I have always considered it. One of the main reasons for the suburban designation was due to being inside city limits of an actual incorporated community. I have never considered it as a requirement to be near a big city. I have just always thought of it as a middle ground, not as develoeped as urban, so less than urban, but not rural.
Some food for thought:
Rural
Definition - of or relating to the country, country people or life, or agriculture
Middle English, from Middle French, from Latin ruralis, from rur-, rus open land
Generally speaking when people talk about rural they are referring to the countryside, not the town.
Suburb
Definition -
1a : an outlying part of a city or town
b : a smaller community adjacent to or within commuting distance of a city
c suburbs plural : the residential area on the outskirts of a city or large town
Middle English suburbe, from Anglo-French, from Latin suburbium, from sub- near + urbs city
Commuting distance is pretty subjective. There are many people living around big cities that commute more than an hour to get to work in the city. There is a city over 50,000 people about an hour and a half away. Does that qualify this small town as a suburb of that big city? If not then I don't think an area like this would fit either definition.