AnyOne Can tell me what's different about MXMemoryMetric and phys_footprint? Which is the really one dicide the app should be killed by OOM?

I am trying to access the cumulative exit counts through the MXBackgroundExitData. I do this in the AppDelegate and upload it to a database for viewing. However, all of the counts are always 0. I tried terminating the application through the app switcher which should trigger a normal exit but the cumaltiveNormalExitCount stays at 0. Should I be accessing this data in a different class?