"frame_id" assignment

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I would like to know the semantic behind the following frame_id assignment:

cloud.header.frame_id = "/map";

Which of the following (or both are) is correct:

1. We want to express our cloud values in the world(/map in this case) frame.
2. We want to express our cloud values with respect to the origin of the world(/map in this case) frame.

I just couldn't see the difference between those two, and at the moment, I presumed that they both carry the same semantic and effect.

Thanks in advance.