What is disjunctive subquery unnesting

Hello. I'm Yuji.
I'm reading Note#15935253.8. It's about "Bug 15935253  Wrong results from disjunctive subquery unnesting"

But I cannot understand this note because I cannot find any friendly description about disjunctive sub-query unnesting.
What is disjunctive sub-query unnesting?

I read some document to understand disjunctive sub-query unnesting.

disjunctive sub-query unnesting transform query which use OR-conditions and sub-query to UNION-ALL query.
What is difference between or-expansion and disjunctive sub-query unnesting. I think both function are very smiler.