That is an alternate connection for the speaker input. It is a DIN style connection that is run in parallel with the rear plugs, so you can use either connection point based on your preference. The connectors are available on eBay.
BeoSystem 5000 (BM 5000, BG 5005, BC 5000, CD50), BeoCenter 3500, BeoCenter 2500, BeoLab 2500, BeoVox S-45, S-45.2, RL-140, CX-100, CX-50, C-75, 3 MCL82 link rooms, A8, 4001 relay and ambio