Much of the information I found on this subreddit regarding sensors was either guesswork or interpretation that conflicted with the CRB, or not all in one spot. So I studied the rules until they were clear, and I hope to provide others with that clarity. Please let me know if anything needs corrected.
Rating
Higher sensor ratings require bigger housings, but increase your limit on sensor tests, and allow more enhancements on your sensor functions. The rating has no bearing on the capacity value or number of functions.
Housing
You need a place to put your sensor(s). The Sensor Housings table (CRB p446) lists possible housings; the higher the sensor rating, the bigger housing you need. The Handheld Housing and Wall-Mounted Housing listed in the Sensors table are not actually sensors; they really should have been listed in their own Dedicated Housings table to view their capacity variance and cost.
Capacity
Sensor capacity is the amount of capacity required to house it, and has no bearing on how many enhancements it can have. It is also a set value, independent of the rating. A single sensor has 1 capacity, and an array has 6.
Imaging and audio devices, as functions, have a capacity of the sensor rating. For example, a medium drone can have a sensor array of rating 4. This means that either a camera or a microphone that you put on it will have a capacity 4. If you add both, then each can have as much as 4 capacity worth of their respective enhancements.
Single Sensor vs. Sensor Array
Essentially:
6 single sensors: 6 capacity, 6 functions of your choice, cost of Rating x 600¥
1 sensor array: 6 capacity, 8 functions of your choice, cost of Rating x 1,000¥
If you have plenty of capacity for the array, it's nice to have for the extra functions.
Edit 1: To correct Capacity section. Sensor capacity values indicate how much space they take up, and not how much they can hold.
Edit 2: Added a short Rating section.
