Hi @caris, welcome to the forum 
A sensor cannot know for sure that it is measuring the quantity it is supposed to be. Our Ping Sonar can use its profile data to estimate the distance to a reflective surface, but sometimes the data it’s measuring isn’t consistent enough for it to know whether there is in fact a surface to detect, or how far away it is if there is one.
I would recommend reading through
- this thread to understand how an echosounder works, and the factors which influence its measurements,
- this comment to better understand the distance estimation process, and
- this thread to get a better understanding of the confidence value
From there, feel free to ask any follow-up questions that you may have