It is important to understand geography concepts used in the censuses - especially for New England. And the terms and concepts have changed over time. Sometimes looking at instructions for a specific census might help (links in the Quick Link box on the left).
As the Census Bureau says: "Geography plays an important role in Census Bureau activities, including supporting the American Community Survey. It is the framework by which the Census Bureau collects and publishes demographic, social, housing, and economic data. Geography contributes to, and is involved in, ACS sampling, data collection, weighting, and data tabulation activities."
Here are some links to help with geography used in the censuses.