Meaning of socket connector labelled 'A'
There are three sockets:

two are fed from black L-connection
the third one with the switch symbol next to it is fed from orange/red A-connection.

This can be used in conjunction with a switch in such a way that you can switch the one output on and off. Particularly useful with a built in switch right next to the socket I guess.
Otherwise if you don't want to use the separate socket switched, you can of course simply use a jumper wire from black to orange tab, as noted by @troubleshooter in the comments below.