This option rental car will guide you to Costco partner’s car rental such as Avis Costco, Costco Budget Car Rental, Alamo car rental and enterprise. You should select one of these car rental partners, and it will give you brief information about their offers.

http://www.rccarsite.com/costco-car-rental/

I'm pretty sure this is talking about a company car, but a special subset of all company cars, i.e. the pool of vehicles reserved for use by partners of the company only.

Typically, these may be more luxurious models, and may be cleaned and otherwise looked after by the company, instead of leaving it to the individual users to do it, as would normally be the case for ordinary company (pool) cars.

Note that in EN we do make a distinction (fiscally, if not linguistically) between a company car (that is permanenetly assigned to one person, like a sales rep, for example, and which they are also allowed to use for private purposes) and a pool car, which is a vehicle kept centrally on the company's premises and which anyone needing to go somewhere on business can use temporarily.