The Discovery Scanner gives you a "level 1 scan". Level 1 scans tell you the object is there, and some very basic statistics about it - type, mass, orbital properties.
The simple way to remember it is that the Basic, Intermediate, and Advanced system scanners can all scan stellar bodies ie Stars, Planets and Moons etc. A Detailed Surface Scanner is required for more information about the object being scanned inc materials etc.
First Discovered By tags do NOT require the DSS.
First Discovered By tags do not require a DSS, but they do require you to do a surface scan of the star or planet in question - that means flying up to them until you get within scan range (this range varies directly with the radius of the object in question, from thousands of Ls for stars down to 5 Ls for neutron stars, black holes and tiny moonlets) and pointing you ship at them, with the object selected as the navigation target, until the scan completes. The object then changes from "UNEXPLORED" to the object's name. Doing all this without a DSS gives you a Level 2 scan, doing this with a DSS gives you a Level 3 scan.
The Detailed Surface Scanner gives you two things:
- Extra credits. The Level 3 Scan bonus increase can vary anywhere from 50% to 1000%, depending on the "quality" of the planet being scanned - generally, bigger is better. And remember, the 50% First Discovery bonus is 50% of the total scan value, so the difference between a Level 2 discovery bonus and a Level 3 discovery bonus can be considerable. Exploration pays a pittance any way you look at it, but it pays even less of a pittance without a DSS.
- Extra information. The Level 3 scan now tells you the materials present on the surfaces of landable planets, and their probabilities - so if you're hunting specifically for vanadium for a FSD boost, you can now know for certain which planets will be worth hunting on and which will be a waste of time. The rest of the "extra information" you get with a level 3 scan, in terms of atmospheric density and composition, planetary internal structure, etc, is currently not "useful" in terms of in-game relevance, it's just flavour text - but may be useful down the track when we can actually land on atmospheric worlds, or deploy mining rigs for deep mining. But until then, all it adds is a little extra stuff to go "Oooh, Aaah" at.