what is the install_device argument for grub-mknetdir

[Daniel Kahn Gillmor]

according to the man page and grub-mknetdir --help, the usage is:

  grub-mknetdir [OPTION] install_device

But it's not clear what the install_device should refer to, since you're 
really just preparing a directory -- there's no device needed, afaict.

Can someone explain the need for this argument more clearly?  Or give an 
example of what should go there?

Thanks,

        --dkg