Final answer:
A Choropleth Map is a map visualization that uses shading to represent relative metrics, showing variation across different regions.
Step-by-step explanation:
The type of default map visualization that uses shading to represent relative metrics is a Choropleth Map. This form of map uses differences in shading or coloring within predefined areas, such as political boundaries or census tracts, to depict the variation in a statistical variable. This can convey data such as population density, election results, or economic indicators. A factor to consider when interpreting these maps is the Moran's I statistic, which is used to measure spatial autocorrelation or clustering on a choropleth map, illustrating how regions may exhibit clustering if data values associated with neighboring polygons are more similar than with non-adjacent polygons.