The Customer List is where you select a customer to work with, and can edit their contact details, account information, record contact information, and work on their quotes/orders.
The Customer List displays the full list of customers and prospects in alphabetical order. When a customer record has a company name this overrides the person's name so an entry for Mr Bloggs of EQ will be listed under EQ in the list view (by default). You can choose the default order of this list, using System Config > Miscellaneous Configuration >Customer Database.
To open the screen click Customer List from the main toolbar:
Read more:
How to filter the customer list
Customer buttons (the left side)
How to start a new quote from a customer