If the coordinate system is geographic, then you would specify a projected coordinate system to get planar geometry measures.

If your coordinate is already projected, such as in a UTM projection, then specifying a State Plane/Modified Transverse mercator, you would get slightly different planar measures since scale factors of the central meridians differ (utm - 0.9996, mtm = 0.9999 )

If your data were sadly in a web mercator projected coordinate , then the planar measures would be really bad and you should be inclined to pick a better conformal, equal area or equidistant one to get more appropriate values.

... sort of retired...