When starting initial processing, PIX4Dmapper first computes keypoints on the images. It uses these keypoints to find matches between the images. The software runs an Automatic Aerial Triangulation (AAT) and Bundle Block Adjustment (BBA) from these initial matches.
If initial processing has already been done and is restarted, the software uses the existing keypoints and matches and starts the AAT and BBA. If the project type has changed from one processing to another, additional matches might be computed as the matching procedure is different for each project type.