For many people, the word “Volume” is synonym for “the complete disk”. Add “Upload” to it, then “Upload Volume size” is easily interpreted as “the total backend storage quota”.
If there’s confusion about this term, why not rename it? What about Block Container File?
The Upload Volume size (AKA dblock files, default 50 MB) can be changed, the block size (file fragments that are stored in the dblock files, default 100 KB) cannot be changed agter the first backup has been made.
Note that the already uploaded dblock files keep the same size. So if you choosed an initial size of 10 GB, make the first backup and change the size afterwards, you have to download 10 GB when restoring a small file that is uploaded during the first backup operation.