Hi, Consider these simpler examples. They have cooked dinner. They did it at some point in the past.
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AnonymousThis is a symbol for those who have tried escaping."Have tried" is a very indefinite reference to time before (or up to) now. Thus, this tends to suggest that more people might possibly try escaping in the future.
AnonymousThis is a symbol for those who had tried escaping.
AnonymousThis is a symbol for those who have tried escaping.The attempts to escape continued until the time this sentence was uttered or written.
AnonymousThis is a symbol for those who had tried escaping.The attempts to escape continued until some time in the past. (That time is -- hopefully! -- made clear by the context