Scan for duplicate files searches for the duplicate files in the list to scan based on the criteria you have defined.
Once you have loaded a directory you can choose "Duplicate files" from the "Scan for..." button or from the "Scan for..."submenu.
Once you have the list of the duplicate files you can use the "Mark/select Options" window to search inside the list of found files.
Note: the list of files that Doublet Scan displays includes all of the files that have the same criteria as what you have chosen.