The Set CFM Total In Multiple Diffusers command is used to set the total airflow through a selected set of diffusers.
To set the total airflow of multiple diffusers on the drawing, go to
Ribbon: DM HVAC->Diffusers-> 
Set CFM Total In Multiple Diffusers
Pulldown Menu: DM HVAC->Set CFM Total->In Multiple Diffusers
You will be prompted to select the diffusers for which the total airflow is to be set.
Select objects:
The Set CFM Total in Multiple Diffusers dialog box will appear.
Set CFM Total in Multiple Diffusers Dialog Box
Old CFM: The current airflow through the diffusers.
New CFM: The new airflow for the diffusers.
See the CFM Setting Methods section for more information about the Average CFM in Diffusers and Prorate CFM in Diffusers fields.
Setting the Total Airflow in Multiple Diffusers
When the values in the dialog box are set correctly, press the OK button. The new CFM value will be set in the selected diffusers.