Advanced parloe to leave or come back?

Is advanced parole requested to leave or to re-enter the country? For example I leave the states and come back for humanitarian reason or because I have a job here? Or go to school?

Or is it: I am leaving the country because My job requires me to tour in another country for 3 months ( example) , I am leaving the country because I am doing research study on the uganda people? or I am leaving the country because my mom is sick and is going to die?

I dont really understand what the advanced is for, for leaving or for coming back? Or for both? Like we cant leave the country and must ask for permission to do so?? Or we need permission to come back in?

Sorry for the typos. Im on my phone.

Is basically permission to leave the country. If you search this topic has been discussed plenty. The problem isn't leaving, it is returning since AP does not grant you automatic re-entry into the US.

I wonder if its not guaranteed that you'll be allowed back in, then why bother issuing AP in the first place.

http://www.ilw.com/articles/2012,0709-murthy.shtm