Hey shanks,

software appliance:

A software appliance is a software application combined with just enough operating system (JeOS) to run optimally on industry-standard hardware (typically a server) or in a virtual machine.^[1] It is a software distribution or firmware that implements a computer appliance.

Windows Server:

Windows Server is a brand name for a group of server operating systems released by Microsoft. It includes all Windows operating systems that are branded "Windows Server", but not any other Microsoft product. The first Windows server edition to be released under that brand was Windows Server 2003. However, the first server edition of Windows was Windows NT 3.1 Advanced Server, followed by Windows NT 3.5 Server, Windows NT 4.0 Server, and Windows 2000 Server; the latter was the first server edition to include Active Directory, DNS Server, DHCP Server, Group Policy, as well as many other popular features used today.

security appliance

A security appliance is any form of server appliance that is designed to protect computer networks from unwanted traffic.

 Computer [cmdb_ci_computer] is the table for your client Appliance types.

Please find below thread for detail understanding:-

https://community.servicenow.com/community?id=community_question&sys_id=97505a91db4533c05129a851ca96...

Below post may be helpful to you.

How are you handling "Appliances" in your CMDB?

https://community.servicenow.com/community?id=community_question&sys_id=504204bb1b0810d4ada243f6fe4b...

Mark Correct and Helpful, if it helps!!!

Best Regards,

Namrata.