Flow diagram of the XM process. The XM uses the onboard detections and catalogue sources with the latest calibrations and parameters from the previous Gaia DR. The first stage is the detection classifier to distinguish real from spurious detections. It is mainly based on detection information, but uses the position of bright sources to classify detections around them. Then the sky is partitioned into groups of detections and sources. These groups are processed without using any source information to determine clusters of detections that belong to the same source. Finally, each cluster is assigned to an input source, or a new source is created. XM provides a list of matched transits to already known and new sources to the subsequent pipelines, together with a list of rejected transits.