Big Business Old 12 is a version of Single-User for running on older computers. Big Business Old Client 12 is a version of Client for running on older computers.
Big Business Old is 32-bit making it more compatible with:
old Microsoft Outlook
If you are using a new computer, running Windows 10, automatically emailing with the latest Outlook, wanting the best network performance, modern windows, and using maximal memory, you want Big Business 12.
If, however, your computer is older, your Windows is outdated, or Outlook was installed a while back, you want Big Business Old 12. It is the same program, just built with old world craftsmanship.