cancellationAttempt
If cancel was called while the EnqueuedJob is in a non-cancellable state (i.e. Executing), and the implementing class has declared a ExecutionPolicy allowing for it, this will be set in order to signal for cancellation.
It is the implementation's prerogative to check this during execution of the job and cancel itself if able.
This will only return non-null while the EnqueuedJob is in the Executing state, isCompleting is false, and ExecutionPolicy.Cancellation.allowAttempts is set to true.