Customers can be assigned to users to in the Customers tab.
You can add/edit customer form by using drop down list "User" with all Users of the Vendor. By default it's set to Not Specified, because usage of this features depends on Vendors' customer service policy. It's always possible to change value of the assigned user to some other users or to Not Specified. Detail behavior of this feature depends on user role in LAC. Current value of User field is visible on add/edit forms, Customers grid and Customer details page.
For Administrators and Sales:
It only information field who is "responsible" for the customer and usage of this field is flexible for vendor. Both roles can change assigned user to the customer without limitations.
For Reseller:
Reseller account is limited and cannot change value of assigned User for the customer. During creation new Customer logged-in reseller is automatically set as assigned user for that customer and he cannot change this to anything else (Sales and Administrators can). Reseller has access only to Customers that are assigned to him. On all grids and API methods he will be able to see and modify customers that he created and/or where assigned to him by other users with Sales or Administrator role. Even if Reseller created new Customer he can still loose access to him if Sales or Administrator will change assigned user to other user or Not Specified.