Details on Report functions FIRST() and LAST()

Is there documentation for the FIRST() and LAST() functions? I am trying to understand what a SET is and how that gets ordered? I was working on a report where I wanted the Last known value entry for a cumulative chart for the day and that would be the result to show for the date group. So I used LAST() and This works fine, but I noticed when I removed redundant fields. The order looks to have gotten flipped and FIRST() became the last known value for the day. Took me a minute to realize this.

If anyone can shed some details on these functions would be great help in me understanding and using these properly.

Can you share the details of your report (including all calculation fields) as it is currently set up - it might make it easier to explain