Generally, when you set DWG Open Options for units to explicit unit values (for example, to Inches or Meters), these units become the master units when the DWG file is opened. The next smallest units on the units list become the sub units. For example, if Feet are the master units, Inches are the sub units.

However, if the units you select are either the master or sub units of the DGN seed file, both the master and sub units from the seed file are retained. If you want to use the sub units of the seed file as the master units for the file that you are opening, you must change the units in the seed file.