Courtesy means that someone has commented on your thread an would like for you to comment on theirs because they took the time to look at your animations. Courtesy also needs to meet the standards of: good critisicm, meaning that you should tell them what to work on, and their rank. Although courtesy is optional it is highly recomended here because it shows that you are polite and not rude enough to just look at your own comments.
Hopes this helps
