You're not looking very hard in the "Help" section of Eraser, are you?
"Detailed descriptions of the overwriting methods and the reason why one should include the cluster tip area when erasing unused disk space can be found from the Advanced Topics. You may want to learn more about the available methods before using a setting other than the default.
On the bottom of the window you can choose which parts of the unused disk space will be overwritten. By default, all options are enabled.
If you select "Free Disk Space", all available, or free, disk space will be overwritten. On Windows NT and 2000, the unused space on the Master File Table records will be cleared as well.
If you select "Cluster Tip Area", the cluster tips of each file on the drive will be erased. You should not use this option for drives compressed with external software - cluster tip area of compressed NTFS drives can be erased.
If you select "Directory Entries", names of all previously deleted files will be cleared from the file system table. " HTH Pete