configuration of the "Document Class Name" in PD cloud

Summary:

As per the business requirement, we have created two item classes (YSS & NSD) under the Root item class and disabled item creation at the Root level, so that items can only be created within one of these item classes. In this context, what item class should I specify for the "Document Class Name"?

Additionally, once the document class is configured, it becomes greyed out and cannot be changed. How should we determine the appropriate document class in such cases?

Should I create a new Item class called Document under the Root item class? Please let me know what is the best practice.