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Like the usual points rewards system when dealing with credit cards, whenever you make purchases, you will earn points. These points can be used to redeem prizes or can be converted as rebate. This is the same with specialized credit cards with frequent flier membership. This type of credit cards is used by people who travel most of the time through air like business men and women who travel around the world just to meet with their clients. So, they receive rewards whenever they fly.

What is "miles-credit"?

This is the reward points that a frequent flier credit card accumulates whenever it is used by a cardholder. The term may vary from credit card providers but the idea is the same. By being a member or signing up to as a frequent flier, whenever you use your card for travel expenses, you will accumulate reward points. The difference between the regular point system and this is that each dollar that is charged and paid for will be equivalent to one mile. If you have accumulated enough "miles" this can give you a free airline ticket.

A perk when you sign up is that you will not start from scratch. You will be given initial miles credit as bonus. Most vary from 20,000 to 25,000 miles credit so you have something to look forward to.

Should I prefer only one airline for me to have benefits on my credit card?

It would really depend on you as a traveler. If you are a kind of person who prefers one airline because based from your experience they give you the best care then if you are using a credit card, ask your airline what credit card providers they are affiliated with. There are some credit cards that give you more reward points and miles credit when you fly a particular airline. They give you additional discounts with the hotels and restaurants even rent-a-car establishments these airlines are affiliated with. So, when you are loyal to the airline, they will share you the benefits they get from their partners and in this case under hotels and restaurants.

What are generic mileage credit cards?

This credit card is used when you use different airlines. It also gives you rewards and miles credit but unlike those that prefer only one airline, this does not have particular restaurants and hotels you can check-in and have a discount. You could ask your credit card provider though if they have any affiliations with these kinds of establishments. If there's none, then you just need to get more miles.