Any factual photograph fits the illustrative category.
Examples can include photo-journalism, street scenes, industrial scenes, shop windows, garden flowers, domestic animals, sports action, theatre and scientific subjects. In fact, the choice is wider than for any other competition. What is important is that a “story” is being told, information is being given. If the image is also aesthetically pleasing is a bonus, but it is the factual nature of the image which is the more important. A beautiful image will not be rewarded, if it does not give factual information!
Images depicting an activity are appropriate here – people “doing things” .
Some illustrative pictures fall under the title Photojournalism. If you want to enter images in Photojournalism competitions or the Inter Club PJ competitions then you can find the Photographic Society of America’s definition of Photojournalism here