Minecraft wiki entry for Superflat, associated text: a list of one or more block IDs (Java Edition or Bedrock Edition);
The block list is a comma-separated list of block IDs, ordered from layer -64 up; if the entry for a given block has an "*", the number before the "*" is the number of layers to be generated, and the number after is the block ID.
A block can also be repeated over multiple layers simply by repeating the block's ID, e.g. minecraft:glass,minecraft:glass,minecraft:glass,minecraft:glass would give the same result as 4*minecraft:glass.
a valid biome ID (Java Edition or Bedrock Edition);
(optional, not after 1.16) a list of structure generation options.
Structure generation options (described below) may have additional parameters, for example village(size=0 distance=9) (in 1.13+ they have no effect).
It is important to remember that multiple parameters are separated by spaces, rather than commas or semicolons.