Explain new clickable check-box states on to-do items

Previously, when you would click a to-do item in a plan, the circle on the left would toggle between checked and unchecked. Now there are several states: unchecked solid circle, unchecked dotted circle, checked solid circled, and crossed-out solid circle:

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What are the meanings of the two new ones – unchecked dotted circle and crossed-out solid circle, especially unchecked dotted circle? I assume crossed-out solid circle means “don’t do this one”, but then why not just click the “x” that appears on the right on hover, to remove that item from the list:

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Also, when to-do items are completed, it automatically changes to checked solid circle, but you can still change them manually to one of the aforementioned states. What does that signify, or what behavior does that cause? And, you can still hover and click the aforementioned “x” to remove the to do item even after that item has been completed. Does that signal the agent to un-do what it just did?

Why isn’t any of this documented (I could not find it after a thorough search)?