how do they decide where to put bus stops?

Were I am at they have some busses that stop at the next intersection if you ring the bell and others that will only stop at designated stops. I always thought they put stops at places they predict people will want to get on and off like government buildings and major private areas like malls and then strewn ones in between at approximate set distances (every mile or two or several). Then I figure they adjust based on if people ring the bell to get off often.