* Past participle of "see": "I have seen that movie three times." (This is the most common usage.)
* Past participle in a passive voice construction: "The problem has been seen by many experts."
* As an adjective (though less common): "The seen world is only a small part of reality." (This implies the world that is observable or known.)
* Implying having witnessed something: "She's seen better days." (meaning she's had better times)
* With "being": "He's rarely seen around here." (meaning he is rarely observed)
* With a noun: "The sights seen during our trip were breathtaking."
The context will determine the precise meaning and grammatical function of "seen."