client- Client-ONLY mods that are completely incompatible with the server and disrupt its operation must be removed before launching the server.

utilities- Don't focus too much on the gameplay itself; primarily, they function similarly to client-side components but are compatible with—or even mandatory for—the server.

libs- Mods – Library and API ,mods that have no direct impact on Minecraft and serve solely as dependencies for other mods.

optimization- A Minecraft optimization mod: if the optimization mod is incompatible with the server, place it in the client directory instead to make server management easier.

$GameplayDirs$- It is not a matter of a single directory, but of an unlimited number of them: each directory is named after the main mod and contains the mod itself along with its add-ons (mods that serve as add-ons for multiple modifications are placed in the directory of the major mod with which they interact)

other- Mods that don't fit into any specific category, or gameplay mods that lack addons and are too small to warrant their own separate directory.