The Siemens 6ES7971-2BA00-0AA0 is a backup battery for the SIMATIC WinAC product line, specifically the WinAC pro, WinAC pro LITE, and WinAC slot 412/416 controllers. It provides a 3.6V supply in an AA form factor. The battery ships with the connection cable and mounting hardware needed for installation, so you're not sourcing those separately.
The 3.6V output matches the memory-backup rail on WinAC controllers; the AA size keeps the footprint compact inside the control cabinet. The included cable and hardware mean the battery is a drop-in replacement — no adapter or custom wiring needed.