The ultrasound sensor cyberware on page 452 of the core rulebook functions "exactly like the ultrasound sensor", but has a listed rating in the table of one to six.
According to page 445 and 446, single sensors are only available in ratings of 2-8, and are limited to a maximum rating of two for headware, which is the section that the ultrasound sensor cyberware falls under.
I'm confused by this discrepancy. Could someone explain the difference? What rating would an ultrasound sensor implant have compared to one built into headware like a helmet?
Additionally, I was under the impression that the rating of a sensor only functioned as a limit to electronic warfare perception tests, which can, RAW (445), only be used with sensor arrays. Does this mean that the rating of a sensor is completely irrelevant when not used in an array? What purpose does the different rating levels on the ultrasound sensor implant serve? Or what am I getting confused?