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 unique customer_id.
Customer ID, is not the same as a Contact listing ID, as customer IDs are unique to the customer and listing IDs only refer to the listing. A profile might have lots of contact listings (with multiple contact listing IDs), but only one customer ID.
To see all contacts with a customer ID assigned, use the the 'Has account' condition in the customer filter:
Apply the filter, then check the Contacts tab in segment explorer for a full list: