How to use the extract-goal-features.py ?

[Sky-railway]

Thanks for your great work ,I want to reproduce the work and re-train the model.but I do not know what does the 'chunk_index' mean in the extract-goal-features.py? what value should I give to the 'chunk_index' when run the extract-goal-features.py file.
As follows:
parser.add_argument(
"--chunk_index",
required=True,
type=int,
help="the chunk index",
)