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.
Images depicting an activity are appropriate here – people “doing things” although these may also be suitable for the People category.
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