Specifically, it means that more information is required to complete the task (e.g. requesting the filename for saving a file). If the action is literally about opening that dialog (e.g. something like "Show Properties"), the ellipsis is not needed.

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layer8 4 hours ago | parent [-]

The practical use is that the user knows they will still have the opportunity to back out of the operation, and not commit to it by the first click. I don’t think “will need more input” is that useful as an information by itself.

	▲	hunter2_ 11 minutes ago \| parent [-]
		At first I was going to say that the opportunity to back out and the need for more input are identical: if the dialog consists only of a button to proceed and a button to back out, the user needs to choose one of those as input, and eliminating that need for input means eliminating the opportunity to back out. But now I'm thinking that a need and an opportunity are very distinct. For example, browsers used to present a Save dialog during a download: was there a need for input? No, accepting the default filename works, and based on that, they no longer offer the opportunity to choose a filename. Thus, "..." indicates the opportunity, even if there is no true need.