when will Release of purchase order cancelled outcome will trigger

Hi,

I am using standard workflow WS20000075 for PO release.

In standard task TS20000166 outcomes there are 3 outcomes

when i perform PO RELEASE in ME29N following outcomes are triggered

1.Release purchase order ------------------------------------ when user  release of purchase order

2.Release of purchase order cancelled ?

3.Purchase Order Changed Significantly-------------------- when user reject of purchase order

but when "2.Release of purchase order cancelled ? " outcome will be triggered.

please answer .

Thanks,

phani.