The J-Link 6-pin Needle Adapter has been designed to connect J-Link to a PCB which does not come with a mating connector or programming header. The designed pattern with 3 locating pins ensures that the adapter can only be connected in one way. The needle adapter allows manufacturers to save costs and space on their PCBs since there is no need to provide the PCB with additional connectors. It adapts from the standard 20-pin 0.1'' J-Link connector to a 6-pin needle connector.
If JTAG support is required, use the J-Link 10-pin Needle Adapter instead, since the 6-pin Needle Adapter does not provide enough pins for JTAG.
More information is available on the website of Segger.