Alias.yml holds the aliases for all items.
An alias in DE is a word that stands in place for the name of an item.
So, the item names you find in items.yml are called the true, qualified names for items. These are the names DynamicEconomy recognizes and uses for anything regarding items. However, surely you don't want to limit users to only being able to call one name! That's where aliases kick in. You can set up multiple aliases for an item, so when people use the aliasname, DE will find the alias in alias.yml, and the corresponding true, qualified name, and understand what Item you're trying to access.
DynamicEconomy, by default, comes with 983 aliases for various items. For example, take the item DIAMONDPICKAXE for example. DIAMONDPICKAXE is the true qualified name of the item as found in Items.yml. However, you can use the words "dpick", "dpickaxe", "diamondpick", and "278". These four values are "aliases" for the item "DIAMONDPICKAXE".
Here's a copy of 13 aliases from the middle of alias.yml:
So, you can see that the format for declaring an alias is: ALIAS:ITEM where both alias and item are UPPERCASE, and there is one space between the colon and the item name.
NOTE: The item name on the right side of the colon MUST be the true, qualified item name as found in Items.yml. NOTE: If you're using a number alias, you must surround it in apostrophes, as you can see.