Special events allow you to be more flexible with what time the engineers complete the job. You can select a slot, for example, 9 am - 12 pm, but the engineer will only be on-site for 1 hour.
This can be done by selecting 9-12 in the diary, ticking special event, then selecting 1 hour.
The event will appear differently in the diary with a clearer background for the overall time frame, then a darker area for the actual time on site.