Is this a bug in DirectX9 Effect Compiler? "Invalid input semantics - POSITIONT0" - c++

Question

I am trying to compile a very simple Effect Shader targeting DirectX9 and shader model 2. I am trying to pass in directly the screen coords for the vertex Positions instead of multiplying by a world, view, and projection matrices so I use the POSITIONT semantic instead of the POSTION0 semantic in the VertexShaderInput structure. However when I try to compile the shader using
fxc.exe /Od /Zi /T fx_2_0 /Fo simple.fxo simple.fx
I get the following error: "error X4502: Invalid input semantics - POSITIONT0"
It seems to be automatically adding a 0 to the end of the semantic for some reason. This seems like I bug to me because POSITIONT is clear described as a valid input semantic here:
https://msdn.microsoft.com/en-us/library/windows/desktop/bb509647(v=vs.85).aspx#PS
and it is the only one that does not allow extra integers to be appended at the end of the semantic.
EDIT: I have just discovered that if I change POSITIONT to POSITIONT1 it works fine. This seem counter to what the documentation above describes.
Contents of simple.fx:
struct VertexShaderInput
{
float4 Position : POSITIONT;
float4 Color : COLOR0;
};
struct VertexShaderOutput
{
float4 Position : POSITION0;
float4 Color : COLOR0;
};
VertexShaderOutput VertexShaderFunction(VertexShaderInput input)
{
VertexShaderOutput output;
output.Position = input.Position;
output.Color = input.Color;
return output;
}
float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0
{
return input.Color;
}
technique Ambient
{
pass Pass1
{
VertexShader = compile vs_2_0 VertexShaderFunction();
PixelShader = compile ps_2_0 PixelShaderFunction();
}
}

Answer 1

For legacy Direct3D9 there is a special D3DDECLUSAGE_POSITIONT which means to just skip vertex processing. Therefore, you can only use POSITIONT in the PIXEL SHADER input--i.e. it can't be used in a VERTEX SHADER at all.
Change your shader to use POSITION or POSITION0.