who command: What does the comment header exactly represent?

When either the who -a -H or who -u -H commands are executed - and having many users logged through tty and ssh - the command works as expected. I did realise the COMMENT header appears **but** always empty. **Question** * What does the _comment_ header **exactly** represent? **Extra Questions** * From where should appear the data for the comment header? * Where and how to put a comment to see it later there? - it only if is possible or viable through a manual process