In January, Microsoft announced a new feature that would come with Windows 10, v1903 -- Reserved Storage.
According to Microsoft, through reserved storage, some disk space will be set aside to be used by updates, apps, temporary files, and system caches. Microsoft went further to estimate that this reserved space would start at around 7GB and would utilize virtual storage technology to occupy space INSIDE the current C: volume.
Understandably, since then, folks with limited-storage devices, most notably those stuck with 32GB "SSDs", have shown increasing concern that should an Upgrade to Win10 v1903 set aside 7GB of their drive space (assuming they even HAVE that much available) that would leave many of them with little or no space to spare.
All is not that grim however, because Microsft indicated some restriction on this feature:
1) It will come only on PCs that have Win10 v1903 already installed
2) It will only apply to a current PC if you do a Clean-Install.
So, the good news is, that if you're running a limited-storage device in Win10, as long as you do a Feature Upgrade to v1903, and NOT a clean-install, you don't have to worry about MS gobbling up 7GB or so of your very limited disk space to set aside for future use.
If you want to read much more detail about this feature, here is a link to the Tutorial on the Windows 10 forums: