DPAF, Dual Pixel AF is a system that is only functional during Live View, and uses the main sensor for focusing. It is unrelated to the Phase Detect system that uses sensors placed in the floor of the mirror box. As I understand it when in LV mode the camera is able to use the extra AF pixels built into the sensor to perform Contrast Detect AF, even while the camera is recording video. As a system that is essentially performing CDAF it's limits are pretty much the same as far as aperture etc. goes, in that the limit is at the point where there is no longer sufficient contrast to perform measurement.
On checking the 5DIV spec's on the Canon UK website, it has information for the PDAF system relating to the working brightness range: -3 to 18 EV @ ISO 100, and the number of points, their orientation and details of accuracy and working f stop. Interestingly the PDAF system has no real details of its useable ranges.
What is important to remember is that as far as the maximum apertures for the PDAF system are concerned is that they are hard switched by the camera's firmware, so unless you do something like using a non reporting converter it is impossible to test the PDAF system at narrower apertures.
Basically: "what TeamSpeed said". DPAF is phase AF (my earlier comments will make more sense in that context).