3D DGN files consist of a design cube in which you work. The design cube represents a 3D DGN file's total volume, in which points are defined with x-, y-, and z-values, or coordinates. Points can be placed anywhere within the design cube, and are not restricted to a single plane as is the case when you work in a 2D DGN file.
Design cube coordinates are expressed in the form (x,y,z). The global origin in the 3D seed files provided with MicroStation is located at the exact center of the design cube and assigned the coordinates (0,0,0). Any point above the global origin has a positive z-value and any point below it has a negative z-value.