The 6" line that enters the building vertically through the slab is called what? I have always considered it a riser however someone else is arguing it's a standpipe.
Technically, it can't be a standpipe because it lacks hose valves. Riser is the correct term based on NFPA 13, Section 3.5.8, which defines RISER as a the vertical supply pipes in a sprinkler system.
chicopee is correct. I would not call a standpipe a riser, though. I reserve that term for main line feeding sprinklers to avoid confusion.
Contractors will call it all a riser, but most of the Fire Department reviewers I've dealt with know the difference.
As per NFPA-13 clause 3.5.9 (2010 ed.), Riser is the vertical supply pipes in a sprinkler system.
As per NFPA-14 clause 3.3.11 (2010 ed.), Standpipe is the system piping that delivers the water supply for hose connection, and for sprinklers on combined system, vertically from floor to floor.
First you have to check what system you are connecting into that vertical pipe, is it sprinkler or standpipe system or both (combined system), then you may clearly identify whether it is standpipe or riser / feedmain line.
I call it the fire service, as it is the pipe that enters the building. Everything directly after the fire service, but before the feed main - check valve (or backflow preventer, if you have one), shut off valve, alarm check, main drain, main waterflow switch, pressure gauges - I would call the riser, or main riser.