How categories could be established among customers?. (example by cities or districts).

Hey prorock, 

Thanks for your post and welcome to the community!

You cannot assign a category to  a customer, if you wish to group customers you would need to create parent customers and assign sub-customer to those. You can set a customer as a sub-customer by going to the customer > edit and select 'is sub-customer' on the customer information panel.

Thanks