A subscribers Order Sequence number is the number of deliveries they have had during the current subscription e.g. If the customers next delivery is their 3rd, their Order Sequence number is 3 'OS:3'
Order Sequence numbers can be used to identify how long a customer has been a subscriber and often provide the opportunity to reward customers at certain milestones during a subscription.
The order sequence number is stored in the Shipping Address notes field:
When the order is processed and placed into Shopify, this OS number will be placed as a Shopify order note: