When I think of Prowl's emotion filters (end of "Emotions," in "Wanting"), I think of magnetic anomaly detection systems used in air-borne vehicles for water-based hunts, from raw data filtering to rapid assessment. Helps me to think that, but it doesn't belong in the story so it's not going to be stated in any actual fic.
Just how did I come to that idea for comparison? Because of my background, that's how! I did some work on MAD systems. In order for a MAD system to work it has to filter out Earth's natural magnetism variations, solar magnetic activity, ferrous contaminants in the air, and other ferrous things so it produces a readout relevant to the user about the size and depth of submerged metallic bodies.
So how am I applying that here? For Prowl, his filters instead filter out typical emotions (natural magnetism variations), most daily normal emotional changes (fluctuating solar magnetic activity), mild emotional response disruptions for uncontrollable events/mechs (ferrous air pockets requiring rapid re-calculation and data filtering for entering and exiting the pocket to avoid false positives or negatives), and such.
The emotion filter percentages say how much of spark-based emotions are being filtered out: 0% is all emotions from the spark are detected by Prowl because nothing is being filtered so Prowl can't perceive it (like a MAD system giving a positive detection "ping" at every small rock down to a 1000-ft depth), or 100% is virtually no emotion is detected by Prowl because it's as completely blocked from reaching him as can possibly be (MAD system not giving a positive readout even when flying over a sunken cruiser barely below sea-level).
His typical settings are set so he'll only feel (read) the highly elevated results, be it love, hate, excitement, terror, protectiveness, depression, or jubilation, but also because he has no choice. See end of "Emotions" for reason, but otherwise Prowl's default is little-to-no emotion.