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 's' is a Sequence object, and the first chain has ID 'A', both 's.chains[0]' and 's.chains['A']' will index this chain.
See Section 6.19 for more information about Chain objects.Example: See selection.assess_dope() command.