tried to understand the Plan feature

Asked Greg the intended behaviour of the Generate Plan option in the Nightly Planner and why it was not affected by the Reset Filters.

 He made two remarks.

Technically, a filter is a function that removes, or filters out, objects from a list.  These include everything on the second line of controls, starting with the Class filter on the left.

Understood.

On the other hand, the Plan is a feature that organizes the list into an observing plan.

I acknowledged him but explained that when I ticked the check box, it filtered out stuff.

It's a whole feature, an alternative mode for the planner.  [I] direct you to the help for the Nightly Planner.  The plan may not include all of your objects if they can't all be fit into a plan during the time available. 

Uh huh. As I thought. The time values might not allow enough time to view all.

Still. I'd argue that's a filtering behaviour.

Anyhoo. Now I know.

And it's time to re-read the manual.

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I read this in the manual:

Also be aware that some observing lists are best at certain times of the year, and if the moon is up for much of the night, many objects may be filtered out.

Um...