Question: For example in the process to prepare a company to be certificate on ISO 20000, how you classify the documentation that are not controlled by SGTI , but are in the same company. For example if I want to make a reference in my document from others departments the documents are external or I can make the referents with the same nomenclature that my company use?
Based on the details, I assume you are implementing Information Security process and related controls.
I would suggest that you make agreement on company level what document confidentiality levels you will use. In such way you will avoid having (e.g. for same kind of document) different confidentiality levels.