Grade weighting can only be set per School. The weighting will be applied to all marking periods and term exams for that School.
If your School has allocated a certain percentage to a marking period or exam and the teacher does not include that component, the calculation will simply disregard that percentage. For example, it will not put in a zero for a term exam that a teacher does not use; the system will simply ignore that in the calculation.
The School also has the option to allow teachers do use their own customized weighting. They can enable this by checking the checkbox at the bottom of the Manage Weighting screen.
When this is enabled, teachers will be presented with the same menu in the Gradebook > Settings section of the individual group space gradebook.
Again, any category that is not used will not be used in the calculations.