Open transaction treatment is very rare because transactions structured completely openly are very rare. If they happen, it would be in an earn-out situation. For example, if you sell an asset in consideration for 10% of the revenues from the asset,to be paid to you until the time when the S&P 500 index reaches 28,000 - you may get an open transaction since neither the time nor the amount of consideration may be determined at the time of sale.
As I said, it rarely happens. In most earn-out cases, there are pre-determined ceilings on on either the amount ("to a maximum of $XM"), or the time ("until the earlier of the S&P Target Date or the 5-year anniversary of the sale") or both.