The keyword phrase "11 sep usa" functions as a proper noun. It operates as a specific name designating a singular, significant historical event, rather than as a combination of separate descriptive terms.
In this construction, "11 sep" (an abbreviation for September 11) acts as a temporal specifier, and "usa" (an abbreviation for the United States of America) serves as a geographical identifier. When combined, they transcend their individual roles as a date and a location to form a compound proper noun that refers exclusively to the terrorist attacks that occurred in the United States on September 11, 2001. This is grammatically analogous to other historical event names like "Pearl Harbor" or "D-Day," which are also proper nouns composed of location or date elements that name a specific event.
Recognizing "11 sep usa" as a proper noun is the main point because it establishes the article's subject as a distinct historical entity. This grammatical classification allows the content to directly analyze the causes, timeline, and consequences of the event itself. It frames the topic not as a general discussion of a date and place, but as a focused examination of a named event with a defined scope and historical significance, which is fundamental to a well-structured and authoritative article.
