The DME in an effort to try to maintain a proper fuel mixture will make adjustments(trim) the fuel to the rich or lean side to bring the air fuel ratio to where it wants it to be. Usually it uses a kind of look up table 8 bits wide which gives 256 cells to the table. Usually the target is in the center at 128 and the DME will go to higher or lower cells to try to correct (trim) the mixture. I think in DIS or IMPA you can actually watch this as it happens and it should give you the target number and show you how it's compensating. If the compensating goes beyond a normal range the DME assumes there is a problem and it will set a code.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)