The most interesting point to understand how the Decimal economy works.
As you know, anyone can create their own coin if they have something to offer the world. Imagine that it is the 15th century and you are a farmer and you have a barn with apples. You have a neighbor - a hunter from whom you want to buy a duck. To arrange for ducks to be exchanged for apples, you need to bargain properly. This is called a natural exchange.
Let's go back to the 21st century.
Decimal has implemented a natural exchange, with the only difference that the exchange rate of apples to duck is subject to mathematical formulas and changes dynamically, depending on the demand for both apples and duck meat.
Your AAA coins are apples. In the exchange tab, you can see how the value of your coin changes if you decide to buy a cup of latte in your favorite coffee shop that only accepts their own CUP coin. You only have AAA coins in your wallet, but Decimal allows you to convert easily AAA to CUP, ensuring that your coin is fairly exchanged for a coffee shop coin based on the current exchange rate of both coins.
Note that if you change one coin to another, you are also moving the DEL reserve to a second coin. This is the basic rule of money: the greater the demand and reserves, the more expensive the coins are.