SRAM's website has no detail on its products, and online retailers give conflicting information about the PC-850. I contacted SRAM (who, like Shimano, is extraordinarily reluctant to answer paying customers' questions online) and got the following response:
Thanks for your email. I apologize for the lack of response, but this is not a
frequently checked email address. Our website and your local bike shop are the
quickest ways to find information on SRAM products. The PC 850 is listed as an 8
speed chain on our website because it was developed for, and tested with, 8 speed
drivetrains. However, since it's only about a tenth of a millimeter thinner than
many standard 6 and 7 speed chains, it tends to be sold as an effective 6 or 7
speed chain as well. While it's not something we test, it's pretty common practice, as there are very few 6 and 7 speed chain options out there.
So it sounds like you should be fine using this chain with a 6 or 7 speed drivetrain.