Only the states documented are relevant. Extraneous ones that may show up in the Intellisense should not be used.
You can cancel on any state that is not a terminal state. A terminal state would be Cancelled, Rejected, Filled. When in a terminal state there is no further action that can be taken. Any other state means the order is still doing something and you can send a cancel on it.
PendingCancel means it is in the process of cancelling so generally would not require you to send another cancel.

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