Surface biomes are the biomes as they appear on the Overworld. Examples: forest, plains. These do not include the underground cave biomes such as dripstone caves and lush caves.

In 21w37a, it was possible for surface biomes to vary according to height in the world. This is found in the save files, where the biome palette can vary according to height. Example: seed 1, coordinates 115, 65, or chunk 7,4 in region 0,0: the biome switches between forest and plains as the player rises through the world. The biome palette in the save files reflects this.

In 21w41a, this no longer occurs. Surface biomes are the same at all heights. The only variation in biomes with height is the intrusion of cave biomes.

This may be WAI (biomes did not vary with height in 1.17.1 and earlier versions), but this needs to be confirmed.