What's the relationship between FANS and CPDLC?
Someone said FANS includes CPDLC, another said it does not.


It depends on the region and/or airplane generation.

There is the ICAO-compliant CPDLC used in Europe (via ATN-B1), and there is the Oceanic/US CPDLC (via ACARS).


A plane fitted with FANS 1/A+ has the Oceanic/US CPDLC, but not the European one.
New long-range planes like the 787 and A350 use FANS 2/B, which has both CPDLCs.$^a$




$^a$ This integration may be offered as a retrofit if it is possible. For example for Airbus – using Airbus' lingo – the A+B package was first introduced on the A350, then became available as a retrofit on the A380 (see slide below by Airbus).


(Airbus via ICAO)



For a brief historical context, see the history section I put together here.