I know the PAX variant is B748.  What is the Cargo variant?

 

The code is the same.

The IATA code is different though, but can´t remember it on the top of my head.

On Simbrief the designated code is B48F,  Is this just a Simbrief thing or is this the official ICAO code?  There has to be some sort of unique code to distinguish between PAX and cargo in the real world and in our online community.  That's what I'm trying to figure out.

For both pax and freighter variant the code is b748. The same also applies to the b777-200lr and b777-200f which is b77L

Per a company filed Flightplan the equipment suffix starts as B748/H. When I get a PDC Departure clearance we use B748 as well.

https://en.wikipedia.org/wiki/List_of_ICAO_aircraft_type_designators

Nothing found here for the -8F Cargo.  Maybe not updated yet.

It is normal for both pax and cargo variants to share same aircraft designation.  The primary use for these designations is to give air traffic controllers information regarding performance.  The B747-8i performs the same as the 747-8F.  Same, as mentioned, for the B77L (it is both a 777-200LR and 777-200F).

Gents - have a look at the 400s for comparison. Same with the 727s, 757s, and 777 cargo variants.

Nothing really changes to the code unless there is some obvious performance or visual change.

B721, B722 - pax or cargo

B752, B753 - pax or cargo

B77L - pax or cargo

Same here.