The first number is the unit key from the tables in AstorBase.mdb starting with UnitOf... (e.g., for a distance it is from UnitsOfDistance). The second number is the unit type from the UnitsType table in AstorBase.mdb. The third number is the unit precision from the UnitsPrecision table in AstorBase.mdb In the above example we used [3,3,1], which means: Key 3 from AstorBase.mdb UnitsOfDistance table: Millimeter
Key 3 from AstorBase.mdb UnitsType table: Decimal
Key 1 from AstorBase.mdb UnitsPrecision table: no decimals
Result: Even with the model units set to display 2 decimals (2643.68) we can have a shown length rounded to full millimeters (2644).