iirc the nopack idea is from addons that have LoD stubs, like Periodic Table. where it has sub addons that are LoD if unpacked. if left as is when u download it all the subs are loaded when the game starts.
putting a .nopack file in tells wau not to un pack that particular addon as by default wau will unpack any addon flagged for it.
that is .nounpack ;)
.nopack does the reverse when updating or deleting,