In oracle application framework page the OAF DefaultStack region was used to hold regions or items in a stack format i.e. it was used when each region or item 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 DefaultStack in current scenarios.
Corresponding web bean: OADefaultStackLayoutBean (deprecated)