Mobile units are subject to a different EN - BS EN 1866:1999. A complicated rating system exists dependant upon the media (water, foam, powder etc) and whether its Class A or B. CO2 is not covered.
Most specifications for mobile units work on the old fashioned premises of mass - just like with portables in pre-fire rating days x area requires a water extinguisher of at least y litres, x risk requires either a y kg be powder or z kilo monnex or a kio CO2.
With regard to foam branchpipe extinguishers - for decades all non chemical foam extinguishers were branchpipe and over the years went through P, FP & AFFF as the foam compound. Until the last few years all foam branchpipe extinguishers were AFFF unless you bought Angus Alcoseal models with AR-FFFP.
Now the few manufacturers of foam branch extinguishers seem to use FFFP - certainly Thomas Glover, Chubb & Amerex do. UK Fire britannia are the only AFFF branchpipe still made routinely.
Any case where you fill an extinguisher with the wrong agent voids the fire rating, voids any warranty and is in breach of the BS. With pwders this can also be dangerous. Failure to perform adequately would result in liability falling on the person who filled it wrongly.
Most AFFF branch extinguishers got a 13A:183B rating for 9 litres. FFFP models are similar, but can have higher ratings (21A &/or 233B). the property of the foams are different, the FP element of FFFP giving better stability and burnback resistance, plus most of these extinguishers are AR-FFFP so you'd loose your polar solvent/alcohol capability.
Considering that the cost difference between a bottle of AFFF refill for a 9 litre ext is only minutely less than that of a AR-FFP refill there is no real cost advantage - better to stick with the designed foam compound and avoid problems later on.
Don't forget most AR-FFFP branchpipe extinguishers now use 90g cartridges instead of the 71 or 75g used in the old AFFF models - use the wrong cart & you'll get poor performance.
SUMMARY - AFFF will work (not as well) in a AR-FFFP branchpipe - but you shouldn't do it!