The setting under Settings >> Site Wide Settings >> Log In & Checkout > Auto-Merge Computer Cart with Customer Cart Upon Log In controls whether your customers will be prompted to merge these two carts or whether the system will do this automatically for the customer.
When set to NO the customer is prompted when the log in with the following message:
"Do you wish to bring the existing contents from your Shopping Cart into your Account Shopping Cart? Please review the contents of your cart prior to checkout as the contents have been updated."
The way that the contents of the "computer cart" and the "customer cart" are merged is as follows:
When the product IDs are all different between the contents of the "computer cart" and the "customer cart" the contents of the two carts are added together.
>> So if I had 3 Sunflowers in my "computer cart" and 2 Red T-Shirts in my "customer cart", when I log into my account these carts are merged so I now have 3 Sunflowers and 2 Red T-Shirts in my "customer cart".
If the same product ID exists in both the "computer cart" and the "customer cart" the contents of the two carts are merged according to the following rules:
For Basic products and Advanced Variant products: >> If the same product ID exists in both the "computer cart" and the "customer cart", the quantity of the product within the customer's cart is changed to the sum of both quantities.
>> So if I had 3 Sunflowers in my "customer cart" and today had 2 Sunflowers in my "computer cart" when I log in I have a total of 5 Sunflowers.
For products with Simple Variants or Personalization Questions: >> If the same product ID exists in both the "computer cart" and the "customer cart", the products within the "computer cart" (those added on the current visit to the site) will REPLACE those products with the same product ID within the "customer cart". Therefore the products the customer added within their most recent visit will be what is listed within their cart after they log in.
>> So if I had 3 Red T-shirts (a simple variant product) in my "customer cart" and today had 2 Red T-shirts in my "computer cart" when I log in I have a total of 3 Red T-Shirts.
