In the study of human settlements, an urban agglomeration is an extended city or town area comprising the built-up area of a central place (usually a municipality) and any suburbs linked by continuous urban area. INSEE, the French Statistical Institute, uses the term unité urbaine, which means continuous urbanized area. However, because of differences in definitions of what does and does not constitute an "agglomeration", as well as variations and limitations in statistical or geographical methodology, it can be problematic to compare different agglomerations around the world. It may not be clear, for instance, whether an area should be considered to be a satellite and part of an agglomeration, or a distinct entity in itself.