Questions relating to community Syncronisation

Hi All

I have subscribed to some of the Microsoft communites such as this one. I
have noticed that if you right click on the community name there is a
syncronise option.

The options are Do not syncronize, All messeges,New messeges only and
headers

By default the option is set to "Do not Syncronise". However when I click
on the forum name to open it does pull down any new messeges since I last
looked at it.

Can someone explain what the syncronise option does?

Cheers.