Register: If you have a virtual machine were you have already put all of the virtual machine files exactly where you want them, and you just need Hyper-V to start using the virtual machine where it is.
Restore: If your virtual machine files are stored on a file share / removable drive / etc… and you want Hyper-V to move the files to the appropriate location for you, and then register the virtual machine.
Copy: If you have a set of virtual machine files that you want to import multiple times (e.g. you are using them as a template for new virtual machines) this is what you want to choose. This will copy the files to an appropriate location, give the virtual machine a new unique ID, and then register the virtual machine.