These are users that do not have a login and hence will not be able to access Celoxis. You can have an unlimited number of such users. Reasons to add such users are listed below.
At times non-human resources like computers, conference rooms and other equipment/machinery may need to be booked prior to use. They may even have a cost associated with their use. By assigning tasks to these resources Celoxis can determine their availability and adjust project plans accordingly. If applicable, the project cost is also updated.
While planning for a projects one rarely knows who will be working on it. This will depend on availability of a resource at that time. What is known is the job function needed for the task. e.g Java developer, Architect, Supervisor. You can create such resources and assign tasks to them. At the appropriate time these resources can be replaced by specific users. If you specify an approximate cost for these resources, the estimated project cost calculated by Celoxis will be more realistic. The other benefit of this is that you can look at resource charts for these generic resources and ensure that you have the right capacity in the future.