Alternate frame rendering 1 and alternate frame rendering 2 are essentially the same thing. What it does in alternate frame rendering 1 is the driver designates the top card to render all the odd numbered frames 1, 3, 5...ect and the bottom card renders the even ones 2, 4, 6...ect. alternate frame rendering 2 simply reverses the work load wiith the top card doing the even frames and the bottom doing the odd ones.

Split frame rendering is exactly what it sounds like. The driver cuts the screen in half and one card renders the top of the image while the other card renders the bottom. This is ofen the least efficient mode because the work load between the cards is not even and then there is also image overlap.

Single GPU is still actually using SLI but all the frames are rendered and compiled on the primary GPU.

Nvidia recommended simply uses the default method that the SLI profile calls for.

Now there are also more modes when you involve 3 or more cards but they are just more or less variations of alternate frame rendering designating which card render which frames. There is also a mix of both modes in 3 or more GPU configurations.