DAC - Derived Air Concentration. DAC is calculated based off of nuclides in the air and their concentration of uCi/volume. One DAC is equal to 2.5 mREM.
DAC-hr is the amount of time exposed to that concentration.
As a sampler runs, in this case the iCAM, the concentration might change.
We'll use easy numbers for an example. The iCAM is turned on and sees an air concentration 10DAC of Co-60. Over the next hour, the display will trend up and finally show 10DACh after an hour. Then all of a sudden, the Co-60 goes away to 0 DAC. The display will trend down over the next hour until it reaches 5DACh. This is because if you were in the area for two hours, you would have seen 5DACh each hour, on average.
Hope this helps some.