Time spent for a particular visit

Documentation Link - https://marketing.adobe.com/resources/help/en_US/sc/clickstream/clickstream.pdf

I am trying to compute time spent per visit using Click-stream Data Feeds. The page 54 on the documentation says that to compute time spent you should group visid_high, visid_low, and visit_num. But instead for finding the visits and visitors the documentation specifies using combination of post_visid_high and post_visid_low (page 53 of the documentation). Should it be consistent across all the metrics? I am suspecting there is some typo or some major point which I am missing.