Here's what I did. I turned off WiFi on my iMac, created a one-slide presentation in Keynote on my iPhone, opened Finder on the iMac and looked at the iCloud drive. The presentation obviously wasn't there. When I turned WiFi back on the file shows up, with the little cloud with the arrow under it. A few seconds later, the cloud and arrow disappear. I turn WiFi back off and loaded the file into Keynote on the iMac.
Looks like it works the same as Dropbox. Except for photos, which don't count against your storage and are stored locally in resolutions specific for the device.