What does the “Used Space” column in the Storage Summary repositories table represent?

This column is meant to represent the artifact size of a repository according to the representation in the Tree Browser. If you were to sum up the size of all binaries in the Tree Browser in a specific repository, this is the number you will have come up with. 

By having it this way, you can also assess how much space you "saved" by having a checkum-based storage when you compare it with the "Binaries Size" field, which isn't bound to a specific repository. This is due to the fact that in a checksum-based storage, the binary file does not necessarily have a 1:1 relation with the amount of DB references. 

Additionally, it is convenient to know what the export size of the corresponding repository is for situations where you need to have a particular repository exported.