Adobe defined six primary character collections based on the script and region. Each collection has a unique Registry and Ordering string (e.g., Adobe-Japan1). The "F" number is an internal shorthand for these collections.
With modern PDF 2.0 and the widespread adoption of TrueType/OpenType Collections (TTCs) and Font Variation Tables, the need for synthetic F-tags is decreasing. However, legacy RIPs (many still running in newspaper and book printing plants) rely on the old CIDFont + F1–F6 model. cidfont f1 f2 f3 f4 f5 f6 full
Furthermore, as long as Adobe Distiller remains in use for compatibility with legacy workflows, CIDFont+F1 through +F6 will continue to appear. Knowing how to interpret and optimize them remains a valued skill. Cons :