What is the fine-grained topic of the following text: It delineates UA boundaries to encompass densely settled territory, which generally consists of: A cluster of one or more block groups or census blocks each of which has a population density of at least 1,000 people per square mile at the time, and Surrounding block groups and census blocks each of which has a population density of at least 500 people per square mile at the time, and Less densely settled blocks that form enclaves or indentations, or are used to connect discontiguous areas with qualifying densities. Only return the topic.
Defining Urban Area Boundaries Based on Population Density