Service categories are used to group events & conferences together. For example in category 1, a main exhibition may have several smaller related conferences, entering the main exhibition code in the category 1 of the other events would link them together. Or, for example, you could group together events that take place overseas in category 2.
You can assign categories to Events to group them together when they are being set up, see Setting up a New Exhibition or Setting up a New Conference.
Adding New Categories
Service Category 1