Additional free space may be required based on the file type you're working with. It is recommended to have minimum 20 GB of free space on your OS hard drive while working with Photoshop.
So, if the free space in your scratch disk is 10 GB, available space that Photoshop will consider in the scratch disk is 10 - 6 = 4 GB, which may/may not be enough for the current operation being performed. However, this is the minimum space that Photoshop tries to keep free in the scratch disk, and the available space that Photoshop considers in a scratch disk is the 6 GB subtracted from the current free space in the disk. Minimum free space on a scratch disk should be 6 GB for Photoshop desktop.
