Uses of Class

Packages that use PotionEffectType
Classes used to manipulate the voxels in a world, including special states.
Interfaces for non-voxel objects that can exist in a world, including all players, monsters, projectiles, etc.
Events relating to entities, excluding some directly referencing some more specific entity types.
The interfaces used when manipulating extra data can can be stored inside item stacks.
Classes to represent various potion properties and manipulation.