SkyDrive and Windows 8.1 blog
A search for reparse point reveals this link A file or directory can contain a reparse point, which is a collection of user-defined data.
For a plain englis discriptio see this link:
What is a Reparse point? - in plain English
"Offline" is determined by the user or an app trying to open the file. You can also force any folder or file to be offline or online-only by clicking the appropriate action when you right-click the object. The always-running SkyDrive.exe takes care of keeping files up-to-date.
The placeholders (using a filesystem feature called reparse points) are always present on your system. But they take very little space. I have 5GB of content in my SkyDrive and the placeholders for these items take about 23MB on my hard drive.
Right-click your SkyDrive folder and choose Properties. There has always been two numbers in here: Size and Size on Disk. I never understood why there were two numbers prior to 8.1's SkyDrive because I didn't know about reparse points. Size will be how much content you actually have on SkyDrive, and Size on Disk is how much the content is currently taking on your hard drive.
Also, setting an item for offline access does not automatically set that item for offline access on other PCs.
