Which marking is specifically designed to indicate sections of a classified document?

Portion markings are specifically designed to indicate sections of a classified document by identifying the classification level of individual segments or portions. These markings are essential for ensuring that readers can quickly assess the sensitivity of different parts of the document and determine how they should handle or disseminate that information. They allow for greater granularity in the classification process, enabling some sections to be declassified while others remain classified.

Banner markings provide an overarching classification level for the entire document rather than for its individual sections, which is why they do not fulfill the need for indicating specific portions. The authority block serves to identify the authority under which the classification is assigned and does not denote specific sections or their classification levels. Confidential markings simply indicate a classification level but do not specify the portion-related context. Thus, portion markings are the correct answer as they are the only type that effectively marks sections of classified material.