The Sync option is at the top right corner and it helps you create a post and copy the content across multiple pages in the same time to customise each of them. 

When SYNC is ON the content will be duplicated across all the pages selected (two or more). 

When SYNC is OFF the content won't be duplicated across all the pages and you'll be able to customise the content specifically for the page selected. 

Did this answer your question?