What is the unit of the slider value when using "Convert Audio to Keyframe"

The value range of the audio to keyframes is 0 - 50. It does not correspond directly to DB. 17.777 corresponds to -12 DB.  -6DB is about 34. -3DB is about 48. 

The DB scale is not linear. It's logarithmic. Here's the formula:  dB = 20log₁₀(A₁/A₂)

If you need DB info, you could write an expression, but the easiest way to see what is going on is to look at the Value graph for the Slider values. If you need a specific sound level to trigger a specific event, then you need to make a copy of the audio track, open it in Audition, and use the equalizer to isolate specific levels or frequencies. I do that all the time to isolate the base track in music. Sometimes I isolate high frequencies. The modified audio track is only used to generate keyframes.

An easier way to isolate specific frequencies and levels is to use Trapcode Sound Keys. That 3rd party effect gives you VU meters and isolates frequencies. It almost makes modify8ing audio tracks unnecessary for most projects.