Difference between Activating and Starting a scheduled entry

I am checking how scheduling works using ‘OEEDemo’ on version 7.8.2. I am creating schedule entries without any changeover time. All these entries shows in ‘Line Schedule Selector’ drop down on ‘operator_control’ screen of demo project. When any entry is selected from drop down menu it ask for activation. When activated the schedule entry block on ‘Schedule View’ screen turns green denoting the time when entry is activated as the ‘Production Start time’ on the balloon popup. This also loads recipe on the line as I have defined recipes with same name as product code. After that when ‘Start’ button on ‘operator_control’ screen is clicked, ‘Enable Run’ OPC tag sets showing that production run for that entry has begun. Also noticed that, now the time when ‘Start’ is clicked becomes the Production Stat time and time when entry is activated becomes Changeover Start time on entry balloon popup.
My question is what is the difference between Activating and Starting the schedule entry and why changeover time is added to the entry even if I have set Changeover time to be 0.

Thanks in advance.