HLSL Shaders

LOL shaders wtf.

I cant manipulate my texture coords properly.
float2 UV0 : TEXCOORD0;
float2 changedUV = (UV0.x - Scrollu,UV0.y - Scrollv);
Col = tex2D(g_DiffuseSampler, changedUV );                   
fairly strightforward, but not.
So..... basics time. what does TEXCOORD0 pass into 'UV0'

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