I'm new to ROS. As part of my journey to understand services I made some notes. They are based on the addtwoints example. I've cleaned them up and posted them here: https://squidoodler.com/ If anyone has time to look them over (its just one page) and tell me if I've understood things correctly I'd be really grateful. I was confused by all the similar sounding labels, so I've renamed things to try and make it clearer (to me) what their role is. I also changed the example slightly to calc_area (mostly because it made the labels shorter!)