Hey @LearnThenShare_JC
The Floor Order field is simply a LONG field nothing else special. It does not need to be set up as a required field and it can accept NULL values.
The only behaviors that happen when the Floor Order field is created on both lines and polygons is that when running the Duplicate Parcels tool and using the Floor Order field as the Update Field, the lines will be duplicated for each iteration. The behavior for Duplicate Parcels when using any other field the lines will not be duplicated, only the polygon.
So to answer your questions individually:
1. The field can be empty (null) and back-filled
2. The 3D visualization is up to you how NULL Floor Order values are presented. You can set up the extrusion method however you please.
3. Long does not have to have a DEFAULT value. It can be null. You can set up a default value if you like meaning that all parcel polys and/or lines can always default to 0. But that is not required.
4. We cover strata parcels in this MeetUp briefly: https://community.esri.com/t5/arcgis-parcel-fabric-videos/meetup-introduction-to-record-driven-workf...
You can also find more info here: https://pro.arcgis.com/en/pro-app/latest/help/data/parcel-editing/createstrataparcels.htm
