When starting initial processing, Pix4Dmapper first computes keypoints on the images. It uses these keypoints to find matches between the images. From these initial matches, the software runs an Automatic Aerial Triangulation (AAT) and Bundle Block Adjustment (BBA).
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 from one processing to another the project type has changed, additional matches might be computed as the matching procedure is different for each project type.