The TRG_ROI precinct sets the ROI mapping in the GEM. In normal operation the ROI mapping details which tiles are associated with with tracker towers. In special acd_as_trigger mode the ROI mapping details the set of tile-tile coincidences with will trigger the LAT. In either case, the TRG_ROI precinct maps the intent from a list of acd channel names to the a set 54 32-bit registers which are interpreted as bit-masks.
| Group | Register Name | Instances | Multiplicity | Subsections | Type | Width (bits) |
| GEM | r | 1 | 54 | 2 | mask | 32 |