It basically is a 'hybrid' of 1.23+ (obviously, most of the community knows about the Xmas release) and Carrotade. Basically it is a program that is (mostly) a server tool, with some 1.23+ commands like /changemusic <musicfile> and some Carrotade commands like /c (changes the level) or /k (a new one, which kills you or any other player of your choice.) Not only that, but it also (just like Carrotade) allows admin access (where players can login by typing /login <password>.) Also, there is a second exe file (the "plusifier") that comes with the utility. The plusifier basically makes a second jazz2.exe (with a '+' in it's name) where this 'new' file automatically loads 'plus.dll' at the startup. (plus.dll has to be in your jazz2 folder, obviously.) But if you're REALLY curious about what JJ2+ does, check out the readme that comes with the utility.
PS: The level list.ini is only there for if you have admins logged in on your server. You won't need it if you change the level yourself, however.