dropbox

The default with Dropbox is that files in your Dropbox folder on your Mac are "synchronized" with a folder in the Dropbox cloud storage. This folder (and subfolders within it) is reproduced on all your devices where your Dropbox account is active.

You can choose "selective sync" where to save space on your Mac, some of the files or folders inside your Dropbox folder are stored ONLY in the Dropbox cloud servers, not on your Mac. You can tell it to download those files to your Mac at any time with a mouse click.

It’s a cloud storage vendor

read

https://www.dropbox.com/features

If you sync the Dropbox to a local dir on your computer it will be included in the time machine backup