Description

This is actually a class, but it has no values; all you'll ever see is the class itself, so it works as if it were a constant. It seems to be a way for an application to return a value while signaling a nonvalue; it isn't an error, and it isn't a failure to return any value at all.

Example

tell application "Finder" to get clipboard -- missing value

In that example, the Finder's dictionary implements clipboard but warns that it isn't yet available; getting its value is not an error on the user's part, but the Finder can't comply either, so the Finder needs a way to reply apologetically, and missing value is its solution.