The hidden numbers are an additional security feature that you can use after checking the other features if you're still unsure that a polymer note is genuine.
The numbers can only be seen by using a small light like an incandescent bulb or a pot light.
WARNING: No light source should be pointed directly at your eye at close range. Dangerous sources of light, such as the sun, laser lights and many LEDs, should never be used to view the feature.
This feature functions properly only when viewed from the front of the note.