Question SV_InstanceID vs unity_InstanceID

what is the difference between getting and using the instance id using:

Code (CSharp):

1. v2f vertexShader (vertexData input, uint instanceID : SV_InstanceID){
2.      float4 data = positionBuffer[instanceID];
3. }

compared to:

Code (CSharp):

1.  v2f vertexShader (vertexData input){
2.      UNITY_SETUP_INSTANCE_ID(input);
3.      #ifdef INSTANCING_ON
4.         float4 data = positionBuffer[unity_InstanceID];
5.      #else
6.         float4 data = 0.0f;
7.      #endif
8. }

I was trying to use Graphics.DrawMeshInstancedIndirect() and a compute buffer, and the second method does not work where the instancing keyword is not defined. However, the second method works with Graphics.DrawMeshInstanced().