...you choose "optionally on" the due date, for a task that does not repeat? I know that for a repeater, it's a way to "auto-forward" the task to the next iteration.
It *appears* that for a non-repeating task, it causes the task to be deleted! I had several items on my task list that are intended to sort of float from day to day until I get to them; they're low priority, mostly, or else semi-dependent on circumstances beyond my control. I set them yesterday to be "optionally" due yesterday - and today I cannot find them anywhere.
Just checking if that is intended behavior, or an oops.
You can also designate a task as optional (abbreviated with a "?"). This acts like "=", but if the date passes and you have not completed the task, the task will be automatically removed, or moved forward if it is a repeating task.
An optional task that has no repeat will be deleted.
This is useful for something that is irrelevant after the date passes. For example "Author book reading at the library". After the date passes, Toodledo will get rid of it automatically since it has no relevance after that date.