The term functions as a noun phrase. In this construction, the head of the phrase is the common noun "year," which is modified by the preceding elements "11 sep."
Grammatically, "11 sep" serves as a compound adjectival modifier. It is composed of a cardinal number ("11") and a proper noun ("sep," an abbreviation for September) that together act as a single descriptive unit. This modifier specifies which particular year is being referenced by associating it with the significant historical event that occurred on that date. This grammatical structure is known as a noun adjunct, where nouns or a combination of words function as an adjective to modify another noun.
Understanding this term as a noun phrase is crucial for its application within an article. It allows the phrase to be used as a subject ("The 11 sep year changed international relations."), a direct object ("Analysts study the 11 sep year."), or the object of a preposition ("Debates continue about the legacy of the 11 sep year."). This function enables the writer to treat the entire year, defined by the pivotal event, as a distinct concept or historical period for analysis and discussion.
