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com.unity.render-pipelines.high-definition/HDRP/RenderPipelineResources/CameraMotionVectors.shader

             velocity.y = -velocity.y;
 #endif
             // Convert velocity from Clip space (-1..1) to NDC 0..1 space
+            // Note it doesn't mean we don't have negative value, we store negative or positive offset in NDC space.
             // Note: ((positionCS * 0.5 + 0.5) - (previousPositionCS * 0.5 + 0.5)) = (velocity * 0.5)
             EncodeVelocity(velocity * 0.5, outColor);
         }

com.unity.render-pipelines.high-definition/HDRP/ShaderPass/ShaderPassVelocity.hlsl

     // TODO: How to allow overriden velocity vector from GetSurfaceAndBuiltinData ?
     float2 velocity = CalculateVelocity(inputPass.positionCS, inputPass.previousPositionCS);
 
-    // Convert from Clip space (-1..1) to NDC 0..1 space
+    // Convert from Clip space (-1..1) to NDC 0..1 space.
+    // Note it doesn't mean we don't have negative value, we store negative or positive offset in NDC space.
     // Note: ((positionCS * 0.5 + 0.5) - (previousPositionCS * 0.5 + 0.5)) = (velocity * 0.5)
     EncodeVelocity(velocity * 0.5, outColor);