What are the 3 designs for counterfeit-proof features?

Counterfeit-proof features can be designed in 3 different ways:

1. Open, i.e. can be recognised and therefore verified by anyone
   Example: holograms, use of colour-shifting inks
2. Hidden, i.e. only verifiable by experts with applicable knowledge
   Example: microscripts, guilloches
3. Forensic, i.e. can be used as evidence in court
   Example: special individual markers

[GMP Compliance Adviser, Chapter 21.H]