Sometimes they are used so effortlessly that we would deny using symbols at all. Sometimes they work and sometimes they do not. To communicate complex meanings in icons, you must expose the viewer to a collection of symbols. Each symbol must have a strong, direct association with the desired meaning, not just in your mind but in the mind of the viewer. After you have decided what objects to include in your icon, you must decide how to draw them.
Appropriate details make objects easier to recognize.
Our eyes seek out detail in a scene and detail can draw our eye to an image. However, detail can distract and clutter. Excessive detail makes it harder to see a pattern of relationships. Often a less detailed image makes a better emblem, especially for general or abstract concepts. The best approach is to select a primary style and use it for most of the icons of a set.