DV360 Exchange ID (CM Model) is a Dimension in Google Analytics under the DoubleClick Bid Manager section
DV360 Exchange ID (CM Model) Definition: DBM exchange ID of the last DBM event (impression or click within the DBM lookback window) associated with the Google Analytics session (Analytics 360 only, requires integration with DBM).
Web Analytics & Insights Google Analytics Dimensions DV360 Exchange ID (CM Model)