Looking at your initial question.. looks like that gave you the following.
* The protocol / interface
* The internal structure (Database schema)
Ask yourself the following questions..
1) What is meant by the requirements? What does the requirement imply the software shouldn't do?
2) What does the protocol imply? What standards should be enforced by the protocol? What are common best practices within that protocol? For example, if it's a JSON/REST service, you expect well form JSON objects, with the right type of quotes, etc... What does the SOAP protocol involve?
3) What do you know based on the schema? What limits does this schema give you? What are unique constraints given what you know?