Marking/Tagging/Favourite Video

I am looking to find a way to 'tag' part of some video clips prior to dropping them in the timeline. I have a lot of video clips, some that a quite lengthy. I am aware that you can no longer mark with comments in iMovie and that you can 'Favourite' and 'Reject'. However, I do not always want to favourite the entire clip, or reject an entire clip. Is there any logical way I can do this? You will see from my screenshot that I have actually managed it, but I have no idea how! Is it to do with the length of the video clip? Do I have to press a button in addition to 'Favourite' to tag just parts? The idea is that I can then review videos on a daily basis, marking sections as required, to speed up editing when dropping them into the timeline.