I have done both Row Robins and Round Robins. The Round Robin that I participated in started with a central block of a certain size. If I remember correctly there were 5 or 6 participants in each group, so each person mailed their center to the next person on the list. So then each person would add a row onto the block as it made the rounds and when it was mailed back to the first person, they had a completed top. There were rules for each round so that everyone got the same size top back when the rounds completed. These were not large tops when finished, I think it was the size of a wall hanging when completed.
The Row Robin was a bit different but also a lot of fun! Each person would make 5 blocks of a theme, stitch them together, then mail to the next person who would then add 5 blocks and on down the list. Blocks were to be 12.5". Once again, when the top came back to the originator, they had a complete top! I enjoyed both types, but I think I liked the Row Robin best. Lots of fun and talent!
I'm sure there are other Round Robin themes out there, but these were the two I participated in.