Volume shaders allow you to visualize a 3D scalar data field (e.g. density field) by sampling field values and mapping to color and opacity. You can assign volume shaders to volume shapes (e.g. Arnold Volume object).
Atmosphere shaders are special volume shaders to simulate a light scattering effect in the scene, like fog. Atmosphere shaders have to be set in Arnold Render settings > Arnold Renderer > Environment > Atmosphere field.
Below is a list of volume shaders available in MtoA:
It is recommended that volume shading networks are kept as lean as possible. This is important for render times in the volume context because it is evaluated so often.