In the '$event' property, we are appending the time stamp along with the event name as well.
In the '$event_id', we are setting the value Sales Order number.
We can see the Klaviyo Developer Documents is updated, but in the old document, it was stated that the combination  'event_id' and 'event' should be unique.

My question was pertaining to how to track an Item Fulfillment against a Sales Order.
The Item Fulfillment can contain partial items or the entire items of the Sales Order.

In NetSuite, Sales Order and the related transactions such as Item Fulfillment and Invoice are different records.
For Kalviyo, we could see that we can send different events such as Fulfilled Order, Cancelled Order ( after Placed Order and Ordered Product events).
Is there a way in Klaviyo to link such events to the same Sales Order (since NetSuite uses different records to track the transactions such as Fulfilment and hence they will have different transaction Id but always will be linked to a parent transaction- Sales Order)?
If we use the "$event_id" as the sales order number for the fulfillment, will it replace the items on the order?