Question about the Length of the output of TimeFeatures in test data

[echo-yi]

Hello, I'm following this notebook and having hard time understanding the docstring below, which can be found in AddTimeFeatures, AddAgeFeature, AddAggregateLags, AddConstFeature source codes.

    If `is_train=True` the feature matrix has the same length as the `target`
    field. If `is_train=False` the feature matrix has length
    `len(target) + pred_length`.

I already passed 'target' that is pred_length longer to test dataset than the 'target' in train dateset. Why should the length of the AddTimeFeatures, AddAgeFeature, AddAggregateLags, AddConstFeature output should be len(target) + pred_length, and not just len(target)?

Thanks.