According to Hitchcock, the MacGuffin has no importance other than moving the story forward, and it has no meaning by itself. It does not matter what the "thing" the characters want is, as long as it drives the character or moves the plot forward. In other words, any detail of a story is a MacGuffin if it can be replaced by something else without changing the rest of the story. If changing what the "thing" the characters want changes anything else in the story, then it is not a MacGuffin.