When you talk about someone being able to improve on their credit score, it's only going to be applicable to those who are making use of credit card because anyone who is making use of debit card doesn't have any business when it comes to improving credit score. I'm using debit card.
Credit score can also relate to when one is able to take up loans and this is different from whether you are making use of credit or debit cards. Most financial institutions that are giving loans will want to have a look at your credit score before deciding if you are qualified or not to take up loans.