Functional earth refers to earthing for a particular function, not for safety. The function is usually noise reduction. I would have thought the purpose would be to decouple the DC-side shielding from protective earth as part of a systemic design to eliminate ground loops in the shielding of all signal wiring.

Related:

  1. Noise Reduction
  2. Using Water Supply Pipe to Connect the Earth Wire
  3. How to install an earthing system?
  4. Is water pipe grounding achieved thru pipe or thru water?
  5. Grounding through the earth pin on a 3-pin power plug