Whats this mean, and whats the difference between the two ? Why does PD need to know where it came from ?
The purpose of this dialog is mainly oriented to the networking. The things is that each Network-card has HardwareAddress, which must be unique within one networking environment (subnet). So, when unknow for Parallels VM is opened, Parallels should know whether this machine's Network Card address is unique. When machine was removed from one MacOS and inserted to another (moved), its address is unique and nothing special will be made. And when machine was copied from another MacOS, Parallels must regenerated its NetworkCard's HardwareAddress. This address is visible under VM's config -> Network -> MacAddress.
Thanks for the explanation. I suspected the question had to do with networking, but wasn't sure if that was the only reason. It might be nice to provide this information on the actual panel that appears.