Default values don't apply to creating objects. If you don't provide an optional member in the body of your request, then the returned object also won't have those members.
There are several objects in Test Center that you can update, clone, or both. However, those operations differ in terms of changes applied to an object you're working on.
When updating an object, you only need to provide required, modifiable members. You can omit optional members, but they will be substituted with their default values. Unmodifiable variables are indicated as read-only.
Cloning, on the other hand, is a POST operation that in case of a missing optional member, uses the value of this member from the source object. This means that in the body of your request you provide only members that you want to override. You can use this operation when you want to associate the same test suites with both a requirement and a config version. Test suites can't be used simultaneously in requirements and config versions, and without this feature you'd need to create all test suites again.