This is a standard Python list of all the chains in the model. You can index this list either in standard Python fashion, or by using the one-letter PDB chain ID, for example if 'm' is a model object, and the first chain has ID 'A', both 'm.chains[0]' and 'm.chains['A']' will index this chain.
See Section 6.18 for more information about chain objects.Example: See selection.assess_dope() command.