In Settings / Edit Site Options, you will find the option for split order processing.
The Split Order Processing option allows you to designate an order's line items as belonging to different numbered segments, so that they may be referenced and processed separately. This option is particularly useful if you regularly take orders that contain a mixture of in-stock and out-of-stock products and wish to process and fulfill them on an as-available basis; or if you regularly take orders with multiple Ship-To Addresses, Ship-From Addresses, or Pickup Locations, and wish to process and fulfill them separately.
If this option is on, new orders are automatically given Segment Numbers when they encompass multiple Ship-To Addresses, Ship-From Addresses, or Pickup Locations. If the Automatic Order Status option (Settings/Boolean Options) is on, segments are also automatically created for new orders that contain multiple line item statuses; you may further select whether line items with the same out-of-stock status (i.e. backordered or preordered) are grouped together into the same segment (with out-of-stock grouping), or are given individual segment numbers (without out-of-stock grouping) to facilitate them being fulfilled separately. Segment Numbers are integers, and are sequential, starting with 1, when automatically assigned. You may also assign (or delete) Segment Numbers when editing an order in the Order Management System.
On the Order Detail screen, line items are organized by segment, with options to view the apportioned totals for each; and to change status and process payment (when a credit card is used for payment, and a Payment Gateway is in use at Settings/Compatible Software / Payment Gateways). Orders are also organized by segment in Fulfillment View (Orders / Fulfillment), with Segment Numbers embedded in Order Numbers, appended with a hyphen (e.g. 102301-1). Segment Numbers may also be used to facilitate order handling and processing via Batch Processing (Orders/Batch Process), the Order Status Import (Orders/Import), Excel and Access Order Exports (Orders / Export), TrueCommerce Connect (Settings/Compatible Software), and the XML Tools (Settings / XML Tools). If you are using one or more Online Shipping Tools (Settings / Compatible Software / Online Shipping Tools), shipping label creation is likewise organized by segment.
When an order segment, but not the entire order, is marked as fulfilled, and the Shipped E-Mail option (Settings / Boolean Options) is on, an Update E-Mail is scheduled for sending to the customer. The scheduled date, as it is for Shipped/Sent/Ready E-Mails, matches the relevant Fulfillment Date. However, the Update E-Mail is scheduled even when the Fulfillment Date is the present date, or in the past; this allows you to fulfill multiple segments of an order in close succession without triggering multiple, potentially confusing, e-mails.
Note: Segment Numbers are for internal use only and are never displayed to customers in the Online Store.