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There are three parts to the despawning system, both are meant to reduce mob count and manage lag.
The first part is simple, it just makes it so no spore mobs will spawn if there aren't players in the world. This is meant for servers.
The second part is a basic mob cap that makes it so only 40 Infected can spawn naturally in the dark.
The third part is more complex, but essentially each tier of spore mob has a maximum number of individuals from that tier that can exist in the world. Every minute, the despawning system will count the mobs in the world based on the mob tier. If there are too many mobs from any tier, the system will despawn the ones farthest away from a player. List of tiers/mobs and their max number of individuals:
Infected - 60
Evolved - 35
Hyper Evolved - 20
Organoids - 40
Scents - 30
Some mobs are specifically ignored by the despawning system, via a changeable list found in the config. Scampers, Gastgebers, Specters, Infested Constructs, Wombs, and Proto Hiveminds are all ignored by the despawning system.
Note the Scent cap here is different from the local Scent cap.
Trivia
A common issue with lag from the mod is mob count, with there often being hundreds or thousands of spore mobs. The Despawning System was made to stop the majority of that lag, and to be able to run an official spore Minecraft server.