Is it the case that a field/property of type enumeration has to have the constraint NOT NULL?
I was just thinking than when replicating data from my server to a local database a value of NULL in a field that represents an enumeration can not be mapped to an element in an enumeration.
Not really, you can have nullable enumerated types. But to properly tell from Delphi if the field is NULL or default value, you should declared it in Aurelius entity class as Nullable<TMyEnumType>.