Separate race series that is sanctioned by IMSA. The Rolex 24 is what you are describing; that is a series for purpose-built race cars.
The GT350R-C competes in IMSA's Continental Tire Sportscar Challenge, which is the support race for the Rolex/Tudor races noted above. The CTSCC is for production-based racecars. The races typically air on TV the week after the actual race, and uploaded to Youtube or IMSA TV the day after the TV broadcast. You can usually watch live on IMSA TV online, as well.
Is hard to imagine that the CTSCC can continue to survive if there are only 8 cars competiting in this event and half of these are Mustangs.
Could someone explain why the new GT 350R-C cannot compete in the Rolex in the GT Daytona class? That class has Porcshe GT3s competing in this class. Dave Pericak stated that when Ford Performance designed this vehicle that their target was specifically the Porsche GT3? Any chance that Ford can get their 350GT competiting in this class anytime soon? Ford must be dumping a ton of cash competing in the CTSCC already. Why not just reallocate these resources to the more prestigious Rolex GTD class?