Atoms Crowd needs some assets to build and run simulations:

• Agent Type: an agent type is an agent definition. It is typically a collection of a skeleton, a skin mesh and some other assets or parameters. Once you have one or more agent types registered in the your host application, you can start building a simulation. 
• Animation Clips: An animation clip defines an animation that can be played on an agent. 
• State Machine: State machines are a way for defining some animation states (walk, idle, run, etc) and how you can transition from one state to another. To make things even simpler, you might think of a state machine as a graph having nodes (the states) and connections (connections are the transitions). Atoms usually needs a state machine if you want to change animation during the simulation for an agent.
• Behaviour Trees: Behaviour trees let the user define conditional behaviours for your simulation. With behaviour trees, you can define both simple and complex behaviours depending on certain conditions. Behaviour trees are optional, you should use them only when planning to build some conditional behaviours for your agents.
• Variations: Variations define combinations of different mesh and materials for your agents.
• Locomotion: A locomotion asset is a system for blending automatically animation based on the agent linear and angular velocities.

Atoms might provide some more assets depending on the host application you are using.