The super jet-pack is an upgraded version of the proto-pack. It flies higher and much faster. After the upgrade, the super jet-pack can still be used as a proto-pack.
After Baldino and Twinsen crash the spacecraft in Otringal , Baldino tells Twinsen to go get a can of GazoGem to repair the ship and upgrade the proto-pack. Once Twinsen does so, Baldino finishes the supero jet-pack, which allows Twinsen to cross the river behind the casino, to access the upper city of Otringal, which couldn't be accessed before, because the elvators were out of service.
"It was put together by Jerome Baldino. It now allows you to access new locations and to follow some grades that were impracticable with the proto-pack."