DVMs allow you to choose a feed algorithm.

DVMs are services run by servers that people can offer. They can do many things, but Amethyst and Primal are using them to provide a "Feed Marketplace". You can go to their tabs and see a bunch of options for new feeds. Click and it will load that algorithm.

The post was about the face that Amethyst shows all DVM providers while I believe Primal is filtering to just a few they like.