For users who use add-on domains, are their bandwidth/disk space limitations all the same for that 1 account? Or do they get, for example, say if their plan was 500 MB Disk, 1 GB Data, that they get 500 MB Disk, 1 GB Data Per Added Domain or is it all shared for that one account?

TIA.

If it matters, we're using cPanel/WHM.

It is all shared for one account. :)

Its all shared for that one account. If you want to divide up the space and bandwidth per domain get direct admin.