When using the multi-event cart option, users can register multiple people to bypass the set field limit. For example, say I have one ticket left that is $10, when the user gets to the cart, they can select register for another event. At that point, they can select the same event and register another person and select another $10 ticket. They can continue to register people for $10 tickets during the same session. The checkout process then lets them pay as normal for however many $10 tickets there are.
The multi-even cart should take into account the fields already in the cart when calculating remaining limits for fields.