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Yes, it's yet another buffer. As was mentioned in several other places, a buffer is typically a small chunk of physical memory that is used as a temporary holding area for data as it travels from one piece of hardware to another to improve performance.
DMA stands for Direct Memory Access, which is a technology that allows certain pieces of hardware, usually disk drives, to bypass the processor and dump data directly into memory. This enhances performance since the processor doesn't have to waste time with these data transfers, and so is free to do other things. There are a number of different DMA protocols, the most recent of which is called Ultra DMA. The fastest UDMA protocol right now is Mode 5, which allows up to 100 megabytes per-second transfer speed.
So what is the use of having a memory buffer for data to stop on it's way to memory? To be honest, I haven't got a clue. If I had to guess, I'd say that since DMA channels point to specific chunks of memory that the buffer holds the data until it can get to the chunk of memory reserved for it. But that's just a guess.
In any event, try setting the DMA buffer size to various settings and see if one of them gives you any noticeable boost in performance. If so, then don't worry about why it works and just be happy that it does. Check the online help for any future updates that will be made to this topic when more information becomes available.