In MicroStation V8 it is important to correctly specify the units for a file. Many of MicroStation's tools, such as dimensioning, placing cells and reference files, rely on the file units to work correctly. Unfortunately in many AutoCAD files it is not possible for MicroStation to accurately infer the file units.
There are essentially two types of linear units used in AutoCAD as specified by the AutoCAD "LUNITS" variable. Files with Engineering and Architectural (LUNITS =3 & 4) are implicitly English units, feet and inches, so there units are known. The other types, Scientific, Decimal and Fractional (LUNITs = 1, 2 and 5 respectively) do not imply a specific unit of measurement.
A second value (INSUNITS) was introduced in AutoCAD 2000 to further specify units and allow for the type of automatic scaling that MicroStation V8 performs when "True Scaling" is specified. Unfortunately the Design Center Units variable (INSUNITS) does not exist in pre-AutoCAD 2000 files and is inconsistently set in many AutoCAD 2000+ files. MicroStation V8 allows the user to either use the Design Center Units specification or to specify units explicitly.
An alert box is automatically displayed when a file is opened and the units are not known. There are three circumstances where the dialog will appear.