Both Dropbox Selective Sync and Smart Sync options require the installation of the Dropbox Sync App.
Dropbox Selective Sync allows you to prevent specific folders in Dropbox from syncing to your local device.
Allows you to search for online-only and local files within your local Dropbox folder
Opening an online-only file or folder will download the file/folder to your hard drive and change the smart sync setting to local.
If the file contains sensitive or confidential information, you will need to reset the setting to online only to keep the file from remaining on your local hard drive.