Understanding Order IDs
When a customer places an order on your BigCommerce store, that order is assigned a unique ID number. It is specific to that order and is never assigned again. This is to prevent confusion when customers do not finish the checkout process, and to ensure that payment gateways do not decline the order as a duplicate transaction.
Starting Order Number
By default, your first order will be Order ID 100. The starting order number can be changed for accounting integrations, store migration or other reasons. To change your starting order number, go to Store Setup › Store Settings. Then click on the Miscellaneous tab and scroll down to the Order Settings section.
Deleted Order IDs
Order IDs may be deleted if the customer starts the checkout process over. This can happen if the customer clears their browser cache, closes their browser or temporarily loses their internet connection. Deleted Order IDs can explain gaps or skipped orders in your Order ID sequence.
These instances are documented in your Store Logs.