If the entire plotting area contains valid, unclipped physical coordinates, then the boundary will just be a rectangular box around the edges of the plotting area.
If the Plot is a Plot3D, this method is applied individually to each of the three 2D Plots encapsulated within the Plot3D (each of these Plots corresponds to a single 2D plane in the 3D graphics system). In addition, if the entire plotting volume has valid coordinates in the 3D current Frame of the Plot3D, then additional lines are drawn along the edges of the 3D plotting volume so that the entire plotting volume is enclosed within a cuboid grid.
AST A Library for Handling World Coordinate Systems in Astronomy