The SQL repository’s inheritance support has the following constraints:

All parent item descriptors (item descriptors that are used in super-type or copy-from attributes) must be fully defined by the time they are referenced in the XML repository definition file. They can be defined in front of the new XML file in the same file, or specified in an XML file that is parsed before this XML file.

You should avoid using too many levels of inheritance. Queries against items whose properties span multiple sub-types may require joins of all tables in the hierarchy. If you use these kinds of queries, keep in mind that performance decreases as the number of tables joined increases.