Sometimes this fee is the same as the option assignment fee, but not always. (valuepenguin.com)
Coming down the pipeline, look for higher fees for award tickets — whether booking fees, seat assignment fees, or change fees — and a less generous award chart. (rewardexpert.com)
Another reason is if you are buying from an institution you need to use the double close or simultaneous close because most banks won't allow assignment fees in their deals. (biggerpockets.com)