Now that you think you have the answer, understanding what is going on in memory is important because different PLCs will respond differently depending on whether the IO is synchronous or asynchronous. If the IO is synchronous to the scan then whatever value is in memory at the end of the scan is the value that is transferred to the physical IO. (This is most common and is probably the answer your advisor is looking for.) However, some of the more modern and most popular PLCs have asynchronous IO updates, the physical IO updates based on memory contents on a predetermined rate, even if it is in the middle of a scan. If you don't understand what is happening in memory on one of these machines then a double coil can really cause some perplexing, and potentially dangerous, process behavior.
Ron Beaufort, a forum member, has some videos on Youtube that explain and demonstrate this. Search for his name at Youtube.com to find them.
BTW, welcome to the forum, hope to see more of you. Controls and Automation is a fun and challenging career path so hang in there.