Originally Posted by
Dirtdiver
Yellow=reserve (high or low) asking to be first or last to be called out. Sometimes it behooves you to go high for a good trip to avoid a bad trip, or to fill up.
Although, nowadays all a yellow means is that you've requested to fly first. No more last out. You can request a specific trip too though. If you see a trip that is uncovered, and you don't want to be sitting around waiting to be called, throw in a yellow.