The IN-DPM feature is used to detect an input source voltage that is folding back (voltage dropping), reaching its current limit due to excessive load. When the input voltage drops to the V_IN-DPM threshold the internal pass FET starts to reduce the current until there is no further drop in voltage at the input. This would prevent a source with voltage less than V_IN-DPM to power the out terminal. This works well with current limited adaptors and USB ports as long as the nominal voltage is above 4.3V and 4.4V respectively. This is an added safety feature that helps protect the source from excessive loads.