In certain industries that question is more difficult, such as medicine and education. In other words, if I am the doctor's customer, why does he treat me like a naughty four year old? And why am I trying to please the biology professor, if I'm her customer? The teacher evaluation survey at semester end doesn't solve the problem.
But even in other industries, the answer can be evasive.
As I tell my marketing students, you have two types of customers: current and potential. But maybe we should consider them to be the same. Particularly for those in rural areas, you should answer the question the same way that the New Testament answers the question "who is my neighbor?" You customer is anyone who can easily contact you and can access your product. For services such as computer repair and for cumbersome products such as hay bales, the circle is much smaller than for something you can easily promote on the web and ship via USPS.
The better you treat your customer, the wider the circle. Our own Nauvoo Quilt and Textile Co. ships fabric to every country on the planet, some of them the textile powerhouses. Their role is to mediate between the textile manufacturers who do business the same way as when the industry started in the 1840s, and customers, who want speedy responses and individual caring. If you live in a remote area, you can do the same thing, as long as you have a post office.