A carbon-infused sintered base adds carbon particles to the already-fast sintered material, reducing friction even further for maximum speed. It's the fastest base option you can get on a snowboard. You'll notice the difference on long traverses, cat tracks, and wide-open runs where maintaining momentum matters.
These bases are found on competition and top-tier boards, and they come with premium pricing to match. They still require regular waxing like any sintered base, but they hold wax longer than standard sintered, so you get more speed between tune-ups.
For most riders, a regular sintered base offers plenty of speed and performance. Carbon-infused makes the most sense if you're racing, riding deep backcountry where speed equals float, or you simply want the best of everything on your board. If you're not maintaining your gear regularly, the extra speed won't matter much.