When you save an edited resource, it's added onto the end, leaving the original version behind. Those would be the ones marked "unused" in the list. "Rebuild resources" takes all the latest resources from all volumes (each resource.### being a separate diskette, and a single resource.000 usually being a CD release) and builds a fresh new resource.000 file with them. resource.map tells which resources there are and on which volume they can be found.
For example, in Space Quest 3, you'll see there's three separate copies of Roger's regular sprite, all view.000, marked as being on volume 1, 2, and 3. As it happens, SQ3 came on three diskettes and Roger can be seen walking around on all three. View 7, the shadow he casts in the red tunnel at the beginning, is only on volume/disk 1. That comes out to 251 view resources spread out over three volumes, including duplicates. Altogether, SQ3's resource volumes are 1.82 MB in size. If I were to click "Rebuild resources", this would be reduced to 186 views across one single volume file that's only 1.30 MB. You can see that all the system scripts (everything >900) is also duplicated on all volumes/diskettes.
If your game uses separate heap resources, you really want to extract and include the hep resources for each scr you changed or its proper functionality can't be guaranteed.
