If you run Erase unused space on the drive in question, it will erase the portions of the previously deleted files that happen to be in unused space. The portions that have been overwritten by other files will not be erased but they will have been overwritten at least once.
The combination should be adequate for most normal purposes.
"Unused space " is just that ... no files. This can be virgin disk space (never used since manufacture) or space formerly occupied by deleted files.
In reality, your deleted files remain in this unused space until they happen to be overwritten by another file. Running Erase unused space forces overwriting of all supposedly unused space so you don't have to undelete the file to erase it.