Thanks for your question.
What you are looking to define here is referred to as the order.subtotal_price in code.
This value returns the subtotal price of all the items in the order after both line-item and cart discounts have been applied. The subtotal doesn't include taxes (unless taxes are included in the prices), shipping costs, or tips.
The Shopify Dev Docs have well documented objects and liquid references that you can use to access or pull information via code.
I trust you will find this information helpful, but let me know if I have missed or misunderstood anything and I'd be glad to chat further.