The documentation uses some terminology that is good to know and understand.
A group or groups of systems managed and run uniformly. Types of clusters include HPC, CPU or render farms, parallel storage, web server farms, database and "big data" clusters, etc.
When no hard drive is present in the node. Not to be confused with stateless!
HPC (High Performance Computing)
Typically used to define a particular type of computing utilizing a system larger then the typical personal computer and used for some form of system intensive processing. These maybe either a cluster like system or a large shared memory resource.
A system that has no persistent operating system storage between boots (e.g. originates from hard drives being the persistent storage mechanism). Stateless nodes do not necessarily imply diskless!
VNFS (Virtual Node File System)
The file system template or image used for for booting many systems
This is the actual operating system image that will be provisioned to a system
A directory that contains all of the operating system components that will be used as a template to create the VNFS Image