Q: When configuring OneDrive on Windows 10, I had to select an option to sync all folders or selective folders, what is the change?
A: Starting with Windows 10, build 9879, OneDrive is moving from its placeholder model to selective sync. Prior to Windows 10, you could see your entire OneDrive content; however, everything you could see was actually only available online and placeholders were present on the local file system that when accessed download the data from OneDrive on demand. You could select a file or folder and make it available offline, which would then download the content and make it available even when disconnected from the Internet—thereby also providing faster access. Changes to the local copy would then by synchronized to OneDrive when connected to the Internet. The challenge some users faced is that they didn't understand that the data was actually online and they would try to access data when offline— but the data wasn't available.
In Windows 10, the placeholder model has been replaced with selective sync. With selective sync you select the folders you'll use on a machine and all the data in that folder in then synchronized locally. Folders you don't select won't be visible on the machine and will need to be accessed using an online version of OneDrive.
