Yes, I was referring to the pets allowed in the cabins. UA (and other carriers) restrict the number of pets allowed on any flight within the cabin. That is why you have to book space in advance, to be sure that you are within their allocation.

Here is United's policy on traveling with pets.

Your dog or cat can travel in the cabin with you, accompanying you as checked baggage or unaccompanied as cargo. All options require advance reservations and are subject to a charge.