Below going through each and what they mean.
Appliance Roles (8.0)
Each appliance can perform a particular role or multiple roles. Below is a description of each appliance role as it relates to the environment:
Presentation – The Presentation appliance is the appliance running the web-based interface you use to configure and manage your installation. Only one presentation appliance is necessary per installation, across all configured environments. All management and configuration of Unitrends Virtual Backup occurs through the presentation appliance’s web interface.
Management – Each environment requires one appliance designated as the Management Appliance. This appliance performs inventory and other hypervisor-specific tasks and manages the work of the Engine appliances. Each environment you add to your UVB deployment requires the IP address of one appliance to act as Management appliance. The Presentation Appliance can also be designated as a Management Appliance.
Engine– The appliances that perform the actual backups processing and send data to their configured backup data stores are the Engine Appliances. Engines are be the most common role an appliance will take on in your deployment.