need help for plugin development : $cart vs $cart_prices

[veeco]

Hi guys, i work on my first payment plugin, i'm bit confuse about the parameter on checkConditions: $cart and $cart_prices
i think it little bit overlap since $cart_prices already include in $cart.

What the purpose of this 2 object ? why there are two instead of one ? i want to modify, which object should i modify ?

Thanks