It's my understanding it means bind-on-use. Something that isn't an equipable (wearable) item can still be bound once you use it. If you're talking about pets, I'd guess it means you can sell the item before the pet spell has been learned, but it's not a cageable pet you can re-sell after it's been in your pet journal.
For me, BoU and BoE are interchangeable. Both result in the item becoming soulbound to your character. Bind on Use makes more sense for pets imo, since you technically can't "equip" a pet item but you can "use" it to learn how to summon it.
Feel free to browse through Perks N Peeves, my vanity pet collecting blog!