Relational databases aren't any different in that:
Models still need to be developed from domain, context, use, etc.
Models are encoded in a database's definition language (DDL).
There is one huge difference:
Databases are not designed for interchange of information. It almost always is the case that the average user will not see the data via SQL or views. As such, the structure, complexity, and ease at which the information is obtained is the purview of the developer and not the owner or user of the information.
That is, with XML, there are more observers of the information and its structures.