What are Friend Credits?

What are Friend Credits used for and how do you obtain them in Ark Knights? Once you have the Reception Room unlocked, you can add friends and visit their bases. This will also unlock a system that lets you find for clues. As a reward for doing this, you'll get credits that you can use in the shop.

Details regarding Friend Credits can be seen in the image below. Take note that friend credits have a maximum number so use it as soon as possible.