If you have a large, organised collection — for example, folders for home movies or music, grouped inside folders for genre or date — you might want to add icons to the folders holding the actual movies or music, but not add icons to the folders for genre or date.
An Organised Collection
If you have very few folders, you could simply drag the ones you want to the folder list by hand. Alternatively, click on the button below the list and select "Add Sub-Folders" in the menu that pops up.
"Add Sub-Folders Too" Menu Item
Now, any folder you add to the list will have its contents scanned for other folders within. This scan is limitless — Add Folder Icons will keep going until it has found every single "sub-folder". For very big collections, this can take a while! A progress bar is shown during scanning and you can cancel the process with the "Stop" button; any folders added so far are kept, but no others are added.
Adding Sub-Folders
After the sub-folders are added, Add Folder Icons automatically selects just the folders you dragged on, but not any sub-folders it found within. So, were you to drag some example genre-based folders to the list:
...then everything inside those would be added underneath:
...and afterwards can just click on the button to remove the genre folders, neatly leaving just the collection you actually want to process.
Now you can choose styles and add some icons.
Final Results