Any used space that is not accounted for by GrandPerspective based on the size of all files it scanned is dubbed miscellaneous used space. Basically, it is unclear what is taking up this space. So if there is a lot of miscellaneous used space and you want to find out what it consists of, you need to scan more. More specifically:

• Scan your entire volume, not just one of its folders
• Use the physical file size measure
• Do not use a filter, i.e. do not use Filtered Scan
• Ensure the application has Full Disk Access

Note: there will always be some miscellaneous used space, as GrandPerspective cannot scan all files. However, these are files that you cannot or should not remove, so for helping you free up disk space, it should not matter.