nom_* are actually views not tables, they join a bunch of other tables in a predefined way. I think these are remnants from the Netbackup Operations Manager (NOM) days, back in the day what is now OpsCenter was 2 products - NOM and VBR which were duct taped together to make OpsCenter. These views represent queries that need to be run all the time - for example NBPolicy view joins policy, client and schedule tables to give a view thats very like the Policy section of the NB gui.
I think nb_ tables are a remnant of a time when OpsCenter could connect to other backup products. nb_* tables were for storing info specific to netbackup and I would presume that if you used OpsCenter to collect from Legato or something then these tables would be empty (this is just a guess).
And domain_* is your generic backup info that can be filled out regardless of what specific backup product you used, though i think since OpsCenter went Netbackup only these tables have taken on a lot of NB specific attributes and are no longer much different from the nb_* tables.
The short answer is that these different names dont really mean too much anymore; just like it doesnt matter whether your cli commands start with 'bp' or 'nb'. Its just what happens when you have a complicated piece of software that grew in pieces over many years.
Now - which table should you use? What I do is check them all and see which of them has most of the columns you need and try to use that one to avoid making unnecessary joins. If you use nom_* tables keep in mind that just running 'select * from nom_NBPolicy' is executing a rather large query (152 lines) in the background and can reduce performance.
