I have read that they are like installment sales, but used when the subsequent payments are impossible to assess. But they are rare because the subsequent payment can nearly always be estimated.
Seems to me that there are lots of instances where the subsequent pay would be wholly unknown. (company profits over 10 years, oil extracted over 10 years, cost of removing PCBs from lake...) Why are they rare?
And in what way are they treated differently than installment sales?