Normally, NVRs or network switching hubs are PSEs (Power Sourcing Equipments) and have no consideration about the PoE and DC power because they generates the PoE power using DC power and have no collision between PoE and DC power.

However, cameras are PDs (Powered Devices) and should select one of PoE or DC power if both powers are supplied. So, if both PoE and DC power are supplied to the camera, the camera use DC power first, and send power status message to PSE via signal lines, then PSE recognizes that the camera use DC power and stop supplying PoE power to the camera.