For example, consider the preset code 1;48,250x0,49,78:2;8
It consists of the following elements:
1 – version number.
48,250x0,49,78:2 – comma-separated list of block IDs.
48 – one layer of Mossy Cobblestone on layer 0.
250x0 – 250 layers of air, from layer 1 to layer 250.
49 – one layer of obsidian, on layer 251.
78:2 – one layer of thicker-than-usual snow, on layer 252.
8 – biome ID, in this case Nether.
A block can also be repeated over multiple layers simply by repeating the block's ID, e.g. 20,20,20,20 would give the same result as 4x20.