But what is different? you ask.
I can’t see anything different! you say.
Well, it is still, basically, Viewer 2, but with one major change: MESH!
Mesh is a new feature that has been in the testing for about a year or so that allows designers to use a 3D designing software, such as 3DS or Wings3D, to create a model, much like a sculpted prim, but without the restrictions that sculpted prims have. It is still under development, so expect rapid changes, but some can actually make meshes and upload them right as we speak.
What you need to upload meshes are a SL account with payment info and you need to take a test, showing that you know about the copyright laws. This is to protect LL and the possible creators because meshes, unlike most things you can upload, is so easy to just steal from others. You can basically just download a model from a game and upload it as it is, but if you do, you are violating the copyright laws. If LL don’t know who you are, they will be held accountable, so that’s why you need the payment info on file.
Meshes are very different from the rest of the graphical elements in SL, therefore the old viewers can’t see it. That’s why Viewer 3 was released along with the mesh. Due to ownership outside LL of some graphical elements, they can’t release the source code to the third party viewer creators, that’s why all other viewers will take some time to update. It is said that Firestorm will be able to view meshes within a week and users of Firestorm will be able to upload meshes within a month, thou this is still in the rumor state.
As of now, Imprudence is the only viewer that is based on Viewer 1 that will be able to view meshes, but they are about a year away from getting there.