You might have one customer identifier you'd like to use for primary reporting (e.g. row ID from the ecommerce customer database). Each contact can be assigned a customer_id which you can choose to use as this ID.
This ID is also supplied in the order records when an order is placed by a registered customer (i.e. not a guest checkout) and is used to identify contact.
The customer ID is different to a contact listing ID, because a profile might have lots of ‘contact listings’ (with multiple contact IDs), but only one ‘customer ID’.
Note: It's very important that IDs sent to Ometria do not contain personally identifiable information (PII) (such as email address, phone number or name).