Invisible Fluorescent Fiber is fibers in paper that are invisible under normal viewing conditions, but can be checked for authenticity by viewing the document under ultraviolet (black) light.
This Invisible Fluorescent Fiber feature is effective primarily in document-present situations where the original document can be examined by a trained professional or scanned with a specialized 3-illumination ID scanner and analyzed with adequate forgery detection technology.
Usability in online and mobile onboarding situations is challenging as the examination is made not on the physical document but on a photograph or scanned image of it.
Ref.: https://www.gpo.gov/pdfs/customers/security_glossary.pdf