Categories in the Better Spawner refer to one of two types: block categories and biome categories.
Adding and changing categories is as easy as adding and changing the file, but there are multiple block categories that absolutely must exist. These are
"structure". Most of these are self explanatory except the final three.
"seesSkyException" are the blocks that are obstructions to spawning, but are not considered blocks that obscure the sky. These are things like leaves, as entities that need to spawn and able to see the sky should not be stopped by being under the leaves of a tree.
"air" are the blocks that are treated like air. This means they do not obstruct spawning in any way. High grass and flowers are examples of this.
"structure" are blocks that will be used in conjunction with the Spawn locations Manmade and Indoors to determine if an entity will treat the generated structure as a spawn location or it will default back to Land or Underground.
Note: Categories will take precedence over actual IDs. If you create a biome category called
desert, then references to
desert in spawn conditions will use this category instead of the
minecraft:desert biome that it would use if the category did not exist.