One end of the adapter has standard PC VGA, the other end has Apple's proprietary Apple video connector. These adapters have a bank of DIP switches for configuration, because the Apple connector had a ...
I have an old CRT. I think it's an Apple Multiple Scan 15 (I'm away from home so I can't check). It came with a Power Mac 6100/66 and required an adapter to plug into that odd HDI-45(?) socket.