Device ID Back Geo refers to the geographical constraints of a campaign, based on where users are when the ad is served to them. For instance, one might target everyone who was present at a particular rock concert, but only want to target users who live in the city where the concert was held, excluding any visitors from out of town. The "back geo" will determine the geographical confines of the ad serve according to the campaign goals.
For DID campaigns, we require what is called a "back-geo" and this is the area where the ads will run after the DID audience member leaves the location and goes home. With DID, we will serve to the audience members who have visited the specified list of locations, but the back geo tells us the exact geographic area we can target where these audience members might reside. So we are targeting these DID audience members who live within the attached list of zip codes.