One of ENIGMA's greatest features is its efficient organization of the resources your game uses. Resources are sometimes also referred to as assets.
There are several primary resources built into the engine that are usually authored through an IDE and then stored in your project file. Resources are referred to by their name, which gets mapped to a corresponding variable whose value is usually a raw integer id unique to that resource. For this reason, resource names must be what are known as valid identifiers meaning they must start with a letter and not a number. Although ENIGMA's rules are basically the same rules for C++ variables/identifiers, GameMaker still has the same restrictions.
There are also resources that can be created dynamically through code and given unique identifiers to work like regular resources.