The Match Diffusers command is used to copy the properties of an existing diffuser on the drawing to one or more other diffusers.
To match diffuser properties, go to
Ribbon: DM HVAC->Diffusers-> 
Match Diffusers
Pulldown Menu: DM HVAC->Diffusers->Match Diffusers
You will be prompted to select a diffuser from which the properties will be copied.
Select diffuser to copy properties from:
You will then be prompted to select one or more diffusers to change.
Select diffusers to copy properties to:
Press ENTER to change the properties of the selected diffusers.