The keyword term is an interrogative noun phrase. It does not function as a single part of speech like a simple noun or verb, but rather as a composite unit whose purpose is to ask a question. The core of the phrase is the noun "year," which is modified by the interrogative determiner "which." The preceding date, "11 sep," acts as a specifier, providing the context for the question.
Grammatically, the phrase is an elliptical, or shortened, question. A complete sentence would be, "In which year did the September 11th event occur?" The query omits the verb ("did occur") and the prepositional phrase ("in") for efficiency, a common characteristic of search engine inputs and headlines. The components are "11 sep" (a noun phrase indicating a specific date) and "which year" (an interrogative phrase seeking to identify the temporal attribute associated with that date). Its function is purely informational retrieval.
In application, the phrase's primary classification is based on its function as a query. It is designed to elicit a specific piece of informationa noun. The event referenced is the series of terrorist attacks that occurred in the United States on September 11, and the correct year is 2001.
