Im only guessing here, but terset files are "terrain sets" It is all the information for a terrain type. There are a number of them available to the map makers. I think its all the data for the trees, bushes, farms, villages, and textures like the jungle colors or desert layout and so on. Its the basic info for the map and is not alterable.
You cant change a terset file or anything in it unless your Hitech. When issues are found like the hiding in a building we had recently Hitech has to go in and fix that. Then each map that uses that terset file needs to down load it to your computer when you log in.
Arena setting are the adjustable settings in the game. For the MA only Hitech can change those. In other arenas like the scenario arenas and WWI arenas and the MNM arena, a few trusted players have the ability to change some of the settings. Which ones I dont know but it could be lethality and height of ack in the MNM arena, or which bases can be launched from in a scenario map.
As for the download, I just downloaded the recent terset file and the file was about 350 meg and my download was at 5 meg a second, so just over a minute. If your connection to the server is crappy, it could take longer, much longer. Hitech has no control over that.
