There is no difference - they are two words that refer to the same thing. “Unit” is the historical term and “Resource” is the currently used term by Bocada. “Resource” and “Unit” are terms used to describe the data source that is being backed up, such as a server, hard drive, virtual machine or individual computer.
For more details about resource definitions by backup application please see the Resource Definition Document here: https://www.bocada.com/bocada-resource-definition