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.
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’.
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: