A heavy user uses significantly more system resources than a casual user. Typical usage for a this type of user may be something like the following:
Presence updates equal to or greater than 20 times a day.
Contact list contains about 30 contacts.
Subscribes to the presence updates of all the contacts in the contact list.
Sets up around four conferences or chats per day where each conference has three people in the conference room and lasts an average of 10 minutes, and a message is added to the conference every 1-15 seconds.