When do I use the DC jack provided with IP cameras ?

PoE IP cameras work from 802.3af PoE port where 48V DC is used and a 12V DC jack.

 These are sort of passive redundant supply methods and are useful in certain scenarios:

(1) At times due to corrosion, physical damage, rat bite, or internal circuitry malfunction you may run into a situation that the camera gets powered using DC jack but not with PoE Port. In such access the camera life can be *potentially extended (provided ethernet functionality works) without repair, by using a PoE Splitter, which splits 802.3af into a regular ethernet port and 12V DC. the DC port can be connected to the Camera's DC jack by splitter supplied barrel jack cable, and the ethernet port can be connected to camera using patch chord. However better is to try and get the camera repair first and only if the RMA center cannot fix it, should you fallback on this method. And while installation, splitter should be enclosed in junction box.

The following link may also be useful for addressing this problem in a DIY manner

https://www.use-ip.co.uk/forum/threads/replacing-a-faulty-hikvision-poe-connector.621/

(2) Sometimes it maybe required to install an AP (with dual ethernet ports) and PoE IP camera at same location. In this method, a PoE/PoE+ splitter can be used to split first the ethernet and DC power signal. Then the DC over signal can be split further into 2 parts one powering the AP and one the Camera. The ethernet port of splitter is connected to AP primary port while the camera gets connected to the secondary port of AP. It does not stop and this, we can have 4-way DC power splits  (12V, 2A) and power complicated setups like switch (<5W), Camera (<10W), AP (<10W)  and other device with 802.3at (PoE+).

(3) Replacing existing analog cameras By IP and reusing the Analog system wiring (Ethernet over coax or MoCA, and DC from cable)

Of-course the DC jack can so be used if UPS power is supplied from nearby outlet and only ethernet  networking function is required from nearby point. 

- Suman Kumar Luthra @ APRC-P3 Telecom Sub-Committee