The value for MergeCart is assigned in the CartCreate request.

The value, "True," specifies that items in a remote shopping cart will be added to a customer's local shopping cart when the customer elects to purchase.the items in their remote shopping cart. The default value, False, means that the items in the remote shopping cart are sent directly into the Order Pipeline instead of being added to a local shopping cart.

This parameter is only valid in the US locale. In non-US locales, the parameter is invalid but the request works as though the parameter were true.