Accumulation/Distribution is developed by Marc Chaikin to provide insight into strength of a trend by measuring flow of buy and sell volume.
The fact that A/D only factors current period's range for calculating the volume multiplier causes problem with price gaps. They are ignored or even misinterpreted. True Accumulation/Distribution solves the problem by using True Range instead of only relying on current period's high and low.
In this example you can see when a gap has occurred in Amazon Inc.'s daily chart True A/D has handled it better than Accumulation/Distribution which a bearish close in period's range has caused it to misinterpret the strong buy pressure as sell volume.