Writing an Arnold-based app:

• Rendering API
• Render device API
• Plug-in Loading API
• Memory Allocation API
• Thread Management API
• ASS File API
• Scene File API
• Message Logging API
• Render Statistics API
• Irradiance / Radiance API
• Versioning API
• Unit Testing API
• Licensing API
• File Utils API
• MaterialX API

Core data structures:

• AtNode API
• AtNodeEntry API
• AtParamEntry API
• AtUniverse API
• AtArray API
• AtEnum API
• AtRGB API
• AtVector API
• AtMatrix API
• AtBBox API
• Math Functions
• Constants

Writing plug-in extensions:

• Shader Nodes
• Camera Nodes
• Output Driver Nodes
• Pixel Sample Filter Nodes
• Color Manager Nodes
• Metadata API
• Procedural API
• Volume Plugin API
• Operator API
• Scene Format Plugin API

Writing shaders:

• Shader Globals API
• Parameter Evaluation API
• User-Data API
• Procedural Texturing API
• Texture Mapping API
• Shader Utility API
• Shader Closure API
• BSDF API
• Volume Rendering API
• AOV (Arbitrary Output Variables) API
• Shader Message Passing API
• Sampler API
• Ray Tracing API
• Light Sampling API
• Sub-Surface Scattering API