Due to the object types structure:

1.There must always be a ‘root’ complex type – usually called ‘object’.

2.This root complex type must have one attribute of type xs:ID – usually called ‘id’ that is the object id. This will be the only attribute of type xs:ID in the entire schema.

3.All the other complex types corresponding to object types must descend (directly or indirectly) from this type.

4.All the elements declared in the schema (with one exception, see below) will have a corresponding object type, so they all must have a complex type descendent from the root type.