Here are the data structures with brief descriptions:

CGField	Abstract base class representing a magnetic field
CGField_AsciiMapFactory	Concrete GField reading a magnetic field from an ASCII map and a YAML definition
CGField_MultipolesFactory	Factory class implementing an ideal multipole magnetic field
CGFieldDefinition	Lightweight configuration carrier used to load and configure a GField plugin
CGMagneto	Manager for magnetic fields and their associated `G4FieldManager` objects