The bedrock-samples-1.21.0.3 example addon pack has a "spawn_category" in the description of the behavior pack > entities > entitiy.json

There's no mention of this spawn_category anywhere online, in the Minecraft Bedrock Stable APIs Reference Documentation, in the entity creation guide, anywhere. However this spawn_category field definitely seems to have some affect on weird spawn mechanics, such as where the mob spawns and possibly the overall limit of the mob. 

https://learn.microsoft.com/en-us/minecraft/creator/reference/content/entityreference/examples/cliententitydocumentation/cliententitydocumentationintroduction?view=minecraft-bedrock-stable

https://learn.microsoft.com/en-us/minecraft/creator/reference/content/entityreference/examples/cliententitydocumentation/datadrivenspawning?view=minecraft-bedrock-stable

Please update this so we know how it related to behavior pack > spawn_rules > entity.json