What determines the story arcs that are in the order. I see some two part story arcs like Green Goblin vs the Crime Master, but longer stories like Greymoor Castle, Titanium Man, or Dr. Strange vs Doramammu aren't considered arcs.

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'The soul knows no greater anguish than to take a breath that begins with love and ends with grief.'- Steven Erikson

Usually, only if it has been assigned a unique name by marvel. There are many multi part strories, but most are not identified by a unique name, unlike say the 2 par "death of gwen Stacy". If I have missed some let me know. The only litmus test is has it been identified as a story arc by marvel, and not jut a story title inside the comic book.

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Oh, gotcha. I didn't know all of those had been given official designations. Cool.

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'The soul knows no greater anguish than to take a breath that begins with love and ends with grief.'- Steven Erikson