Typically in Sony model codes, /b indicates that it is the black color model. Though it's not significant in the RX100III as it only comes in black, Sony appears to still use their internal coding and includes the /b in the title. When looking up the camera at B&H, the 'RX100III' is B&H's own code for the model, while 'RX100III/b' is Sony's official model number for the same camera. Where you see it come into play is when there are models with different colors - the A6000 for example comes in an ILCEA6000/B, ILCEA6000/W, and ILCEA6000/S for black, white, and silver. Then you get into Sony's other codes when you go to their interchangeable lens cameras - like /L which means it includes the kit lens, and /BCOS for two lens kits, and so on.