In oracle application framework page the OAF DefaultFormStack region was used to hold regions or items in a stack format i.e. it was used when each region or items needs to come in Oracle apps framework page one under another (stacked) either to the left side of the page or right or center as per the requirement.
It is a deprecated region style.
stackLayout should be used instead of DefaultFormStack in current scenarios.
Corresponding web bean: OADefaultFormStackLayoutBean (deprecated)