DropStuff 3.5's main addition to StuffIt 3.0 was that, in Mac OS 7.1 or later, you can drag any amount of files and folders in the Finder onto the icon for DropStuff or an alias to it instead of having to use the standard dialog and DropStuff will create the archive with the files dropped onto it's icon. With the introduction of this feature, it was also the first version that was named "DropStuff".
DropStuff will only run on Mac OS 7.0 or later because, for some reason, it HAS TO keep checking for AppleEvents regardless of whether the system software it is running on supports AppleEvents or not. However, the "Expander Enhancer" bundled with DropStuff that the equivalent version of StuffIt Expander can also expand .zip, .arc, .gzip, .z, .pkg, .uu, and StuffIt SpaceSaver archives with, for StuffIt Expander 3.5, will work on System Software 6.
