As mentioned in the configuration documentation, configuration files are just Protocol Buffers objects described in the .proto files under research/object_detection/protos. The top level object is a TrainEvalPipelineConfig defined in pipeline.proto, and different files describe each of the elements. For example, data_augmentation_options are PreprocessingStep objects, defined in preprocessor.proto (which in turn can include a range of other possible objects for different preprocessing tasks). The meaning of each object and field may or may not be obvious or well-documented, but you can always refer to the source code to see exactly how each value is being used (for example, check preprocessor.py to understand how data augmentation is done).