Clock source

[musickid]

when i connect my dac to my imac i am presented with two options regarding clock source. the first is the standard set up on audio midi set up where the clock source is greyed out and states clock source default. the second is when adding the dac as a single aggregate device and here the clock source states the name of my dac and also greyed out. if i want to ensure the dac clock is used am i right in thinking both options do this. . MANY THANKS