PID values above max?

APM Community,

 I have tried to find some information regarding PID tuning Min and Maximum values. 

Specifically I am looking for information about the following:

What happens when a PID setting is above the maximum value?
-For example
Rate Roll P is above its maximum value?
Stab Pitch above 12?

Are the values above maximum ignored? If i set a stability value of 15 does it ignore the 15 and assume 12?

Why is the limit 12 for stability tuning?
Why does autotune often set values above 12 for stability? I’ve seen it as high as 14.9 sometimes 19.

I want to fully understand the pixhawk tuning parameters so that I can tune each setting to the best of my ability but I don’t seem to see any answer regarding values beyond max.