In Circuit Diagram, all components are loaded from files in the component directories. There are two formats which components can be stored as - XML format or binary format. XML component files are human editable, while binary files cannot be edited directly. Binary files are made by compiling XML component files.
An XML component file contains one component. However, components may contain configurations which enable them to be more flexible. For example, the component file may be called "resistor.xml" and contain the configurations "resistor", "variable resistor" and "thermistor".
If you are not familiar with XML, you may find reading this tutorial useful before continuing.