When a Building is created an independent set of Datum's are created for all the levels in the building. Having these independent Datum's allows individual buildings to be easily created and edited with different floor levels.
These floor levels can be useful for other parts of design as well. For example Columns that surround the first floor of a Building, or a Features used as an entrance canopy.
While it is straight forward to add these additional elements in the correct position at the time of creation, what happens when you want to change the building levels? Do you need to manually update all these other elements?
This is where Datum linking comes in.
Both Features and Columns can be linked to the Datum's within a Building so that any changes to these Datum's will also update the linked Features and/or Columns.
Using Features as an example you can see in the Settings the ability to link the Feature to a Building.
Having linked to a Building you can now set the Top of the Feature to specific Datum's. You can also set an offset.
With these linked in place, you can use the Datum palette to adjust the Datum and see the model updates: