line item vs request level service catalog approval

can s.one tell me what's the exact diffrence between line ite approval and request level approval in the approval tab for an item on service catalog 

alos how can i set an approval notification to be sent to the approval group on line item ,i tried to add an html rule it didn't work ,alos it doesn't exist in the out of the box ,i need to set notification to be sent to prnding approval that's related to the line item approval .