Axe's documentation uses 'buffer' to mean 'a pointer to some data'. Any command that takes a buffer is really asking for a pointer. As long as the data you are pointing to is big enough for whatever operation the command is doing, it is happy.

Buff() makes a buffer by adding bytes to your program. If you make one with a size of 1000, it adds 1000 bytes to your program's size. The lines :Data(0, 0, 0, 0, 0)->X and Buff(5, 0)->X both do the same thing. Buff() is just a convenience if you want a lot of space.
It is a way to get more scratch memory if you run out of space in L1-L6 and don't want to use GetCalc() to create a temporary file to store data in. For example, if you want to store a screenshot (which is 768 bytes) and you are using L3 and L6 for something else, you could use Buff(768)->^oScreenshotStorage to have a place to store your data. Since DispGraph can take a pointer, DispGraph(^oScreenshotStorage) would display the contents of the buffer. (Of course you would probably want to put something there first)

I can give more detailed help or suggestions if you tell me what you are trying to do.

And if it just returns normal data, does that mean that the following hexadecimal, when stored to a pointer, can be drawn as a buffer with a line on the left side of the screen?

[800000000000000000000000 (repeat shown hexadecimal for 63 additional rows, total of 64 rows)…]

Yes. DispGraph(ptr to that data) would display a blank screen with a line on the left side.

In general, unless you are really hurting for speed, you can get away with redrawing everything each frame. Axe's drawing routines (except for text) are very fast.