Service retention is when you like start a service or something or like want to stop/start a service. Not sure. It was mentioned in the tutorial where you have to press it and stuff. I don't think it really does something cuz I don't bother touching them. I just leave them at the default settings.
Here you have detailed instructions. https://www.google.com/url?sa=t&sou...IQFnoECAUQAQ&usg=AOvVaw3pOze4G3oukPk1H5iw-LQ_