I think I have the input and output between vertex and geo shader right, but here is the error:

D3D11 ERROR: ID3D11DeviceContext::Draw: Vertex Shader - Geometry Shader linkage error: Signatures between stages are incompatible. Semantic 'TYPE' is defined for mismatched hardware registers between the output stage and input stage. [ EXECUTION ERROR #343: DEVICE_SHADER_LINKAGE_REGISTERINDEX]

Here are the declarations on the c++ side:

struct Particle
{
	DirectX::XMFLOAT4 InitialPos;
	DirectX::XMFLOAT3 InitialVel;
	DirectX::XMFLOAT2 Size;
	float Age;
	unsigned int Type;
};

enum ParticleType
{
	standard,
	rain,
};

static D3D11_INPUT_ELEMENT_DESC ParticleVertexLayout[] =
{
	{ "POSITION", 0, DXGI_FORMAT_R32G32B32A32_FLOAT, 0, 0, D3D11_INPUT_PER_VERTEX_DATA, 0 },
	{ "VELOCITY", 0, DXGI_FORMAT_R32G32B32_FLOAT, 0, D3D11_APPEND_ALIGNED_ELEMENT, D3D11_INPUT_PER_VERTEX_DATA, 0 },
	{ "SIZE", 0, DXGI_FORMAT_R32G32_FLOAT, 0, D3D11_APPEND_ALIGNED_ELEMENT, D3D11_INPUT_PER_VERTEX_DATA, 0 },
	{ "AGE", 0, DXGI_FORMAT_R32_FLOAT, 0, D3D11_APPEND_ALIGNED_ELEMENT, D3D11_INPUT_PER_VERTEX_DATA, 0 },
	{ "TYPE", 0, DXGI_FORMAT_R32_UINT, 0, D3D11_APPEND_ALIGNED_ELEMENT, D3D11_INPUT_PER_VERTEX_DATA, 0 },
};

My struct that I use and we see the TYPE semantic.

struct Particle
{
	float4 InitialPosW : POSITION;
	float3 InitialVelW : VELOCITY;
	float2 SizeW : SIZE;
	float Age : AGE;
	uint Type : TYPE;
};

Here is the simple vertex shader:

Particle main( Particle vin)
{
	return vin;
}

And the more complex Geometry shader:

[maxvertexcount(6)]
void mainGS(point Particle gin[1], inout PointStream<Particle> ptStream)
{
  gin[0].Age += playingTime;
  
  if (gin[0].Type == PT_EMITTER)
  {
    // time to emit a new particle?
    if (gin[0].Age > 0.002f)
    {
      for (int i = 0; i < 5; ++i)
      {
	// Spread rain drops out above the camera.
	float3 vRandom = 35.0f*RandVec3((float)i / 5.0f);
	vRandom.y = 20.0f;

	Particle p;
	p.InitialPosW = float4((emitterPositionW.xyz + vRandom),1.0);
	p.InitialVelW = float3(0.0f, 0.0f, 0.0f);
	p.SizeW = float2(1.0f, 1.0f);
	p.Age = 0.0f;
	p.Type = PT_FLARE;

	ptStream.Append(p); 
      }
 
      // reset the time to emit  
      gin[0].Age = 0.0f; 
    }

    // always keep emitters
    ptStream.Append(gin[0]);
  }
  else
  {
    // Specify conditions to keep particle; this may vary from system to system.
    if (gin[0].Age <= 3.0f)
      ptStream.Append(gin[0]);

  }
}

GeometryShader gsStreamOut = ConstructGSWithSO(	CompileShader(gs_5_0, mainGS()),"POSITION.xyzw; VELOCITY.xyz; SIZE.xy; AGE.x; TYPE.x");

Anyone got an idea why I am getting this, I've seen this error before, but not when both are using the same input/output structure.

Solved... The problem was nothing here! As I expected.

I accidentally swapped the stream geo shader with my expansion geo shader at compile time. LOL