Two statements about the Order Summary object that are accurate are:
It does not exist without the original Order object.An Order Summary is a record that represents the financial summary of an order that is received from an external system, such as B2C Commerce or B2B Commerce. An Order Summary has a master-detail relationship to the Order object, which means that it cannotexist without its parent Order record.
It is read-only for financial order data. An Order Summary contains financial order data, such as total amount, tax amount, discount amount, etc., that are populated from the external system when the order iscreated. These fields are read-only and cannot be edited in Order Management.
Verified Reference: https://help.salesforce.com/s/articleView?id=sf.order_management_order_summary.htm&type=5

When a customer applies a discount when placing an order, an order adjustment group summary is created. An order adjustment group summary is a record that represents a modification to the order amount,such as a discount, a surcharge, or a tax. An order adjustment group summary can have one or more order adjustments, which are the individual modifications that are applied to the order or the order items. An order can have one or more order adjustment group summaries, depending on how many types of modifications are applied to the order. Reference: Order Management Objects, [Order Adjustments]

The number of Fulfillment Orders created depends on three factors: Fulfillment Location, Delivery Method and Recipient Address. These factors determine how the Order Items aregrouped and allocated to Fulfillment Orders based on routing rules and inventory availability. For example, if an order has two items that can be fulfilled from different locations, or have different delivery methods or recipient addresses, then two Fulfillment Orders are created for that order. Verified Reference:https://help.salesforce.com/s/articleView?id=sf.om_order_fulfillment.htm&type=5