Primarily AC use, What Does it Actually Do!?

My understanding, primarily AC use is a charging scheme that can prolong the health of your battery.  It will stop charging when the battery power level reached around 85%.  But it will reporting that the battery is fully charged (100%).  That way, it prevents top off charging.

Fast charging and prolong a maximum charged level often shorten the life of lithium battery.  Also if you install a long life battery (from precision line), charging options will not be available.

I believe you are either right or my new battery is already faulty...I tried "Primarily AC use" and then when it reached 100% removed the cable, the laptop reached approx 25% (but in this mode it is actually 15% right??) and after very short time it suddenly it went to 7% and turned off...25 - 15 = 10 (I used the laptop a little and it reached 7), anyway weird and unclear mode   will update when I better check it out.

I looked in the bios and the battery is charged like windows says, is this -15% actually kept so secret?

No, it's no secret that primarily AC use mode extends battery life by lowering the charge threshold.  Here is the description from battery settings: 

Found similar answer from another Community Accepted Solution

The practice is widely used according to this report.

correction:

I tried again I can confirm that with this mode when  on battery and you reach around 30% the battery suddenly drops the amount of about 20% that is it goes to 10%.

What makes this mode difficult to understand is that the battery gets charged fully to 100% but taking into account this sudden drop when it reaches about 30% it tells there is something unclear about this mode