I'm reviewing a tutorial on using textures and I see that the vertex shader has this input declaration where the position is float4

struct VertexInputType
{
    float4 position : POSITION;
    float2 tex : TEXCOORD0;
};

But when they go over uploading the data to vertex buffer they only use a float3 (Vector3) value for the position

// Load the vertex array with data.
vertices[0].position = D3DXVECTOR3(-1.0f, -1.0f, 0.0f);  // Bottom left.
vertices[0].texture = D3DXVECTOR2(0.0f, 1.0f);

vertices[1].position = D3DXVECTOR3(0.0f, 1.0f, 0.0f);  // Top middle.
vertices[1].texture = D3DXVECTOR2(0.5f, 0.0f);

vertices[2].position = D3DXVECTOR3(1.0f, -1.0f, 0.0f);  // Bottom right.
vertices[2].texture = D3DXVECTOR2(1.0f, 1.0f);

The input layout description declared also seems to match to use a float3 value

polygonLayout[0].SemanticName = "POSITION";
polygonLayout[0].SemanticIndex = 0;
polygonLayout[0].Format = DXGI_FORMAT_R32G32B32_FLOAT;
polygonLayout[0].InputSlot = 0;
polygonLayout[0].AlignedByteOffset = 0;
polygonLayout[0].InputSlotClass = D3D11_INPUT_PER_VERTEX_DATA;
polygonLayout[0].InstanceDataStepRate = 0;

So does this mean that shaders will automatically default "missing" values to 0 or something of the like? If so is this frowned upon?