Shader model 6.6 adds support for the following.

• New Atomic Operations (64-bit and bitwise floats)
• Dynamic Resources (direct access to heap resources from the shader)
• New IsHelperLane() intrinsic (available in older shader models as well)
• Derivative Operations in Compute, Mesh and Amplification Shaders
• Pack and Unpack Intrinsics
• WaveSize
• Raytracing Payload Access Qualifiers

An overview and detailed specifications on each feature can be foundhere.