Rumors have been swirling around for a few months now that OneDrive placeholders would make a return in the Redstone update for Windows 10. Twitter user WalkingCat has uncovered the first signs of that actually happening.
As with all early signs like this, we always advise readers to take them with a grain of salt. While it's clear that it has something to do with cloud storage and with something called "Placeholder", that is quite literally all we know for sure.
Keep in mind that early builds of Windows 10 Mobile had an Android subsystem built into them; however, the RTM build did not. It's still very early days for Redstone, so it's important to not jump to conclusions.
If you're unfamiliar with OneDrive placeholders, this is the technology that was implemented with Windows 8.1. Users could go into File Explorer and see their entire OneDrive. Some files were stored online-only and some were available offline. When attempting to use an online only file, the system would download the file for use. Users could also right-click a file or folder and choose to make it available offline or online-only.
Microsoft removed this capability in Windows 10, making OneDrive work more like it does on Windows 7 and OS X. Users would need to choose which folders to sync and then only be able to see those files and folders in File Explorer.
Microsoft had said that they made the decision because users were confused as to why they couldn't access their online-only files while they were offline. They did promise to build an "alternative" to the deprecated placeholders before the end of 2015; however, they never did.