A working plane is automatically added to a room once it is created. You can define a utilisation profile for this room in order to follow standards and check if the required illuminance is reached. 

If you wish to create a zone within a room with a different level of illuminance, you can create an "Assessment zones".
Here you can define the size and position of the respective zones within the room, allocate a distinctive name to each and define an own utilization profile.