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Terminology
Plot area
A plot area is any area that PlotSquared will manage/handle. If this is an infinite plot world, the entire world is considered to be a plot area. If you use plot clusters, then only part of the world will be a plot area, and anything outside this area will not be handled by PlotSquared.
Clusters
Clusters can be created within existing plot areas, or they can be created in a previously non-plot world, which will in turn create it's own plot area.
Road
A road is what separates each plot, and includes the wall around each plot. Attempting to get a plot at this location will return null.
Plot
A plot can be claimed or unclaimed. Getting a plot at a location where one isn't claimed will return a new unowned plot object.