first, we need a pattern to descripbe this situation, then we implement the customer's specification,
if you have a server, you write a application which is responsible for answering your customer's request , you have 3 customers now, they have same interface but a little bit different implementation, when you design the software, you know you will have more customers in future, if you need modify a lot of code when you add a new client, sometimes you can't remember where you should add code for the new client, you find you should design your project well at the first
that's it.