AP itself is not high/low density but it is the area in which it is deployed. Like a conference Hall, corridor, classrooms, offices in high-rise tower buildings, these are all high density environment as a lot of people are present in a small area. In such case, high density APs like 7050 is deployed to facilitate and serve maximum users. These APs have high processing and are capable to serve multiple requests simultaneously.
Please let me know if more details are required and I'll be happy to share.