Hello ruyi7952,
exportMatches() function saves the tie points between the images. They are defined on the 2D images.
PhotoScan.PointCloud is the name of the Python class. It's not related to the actual project, so if you need to save the sparse point cloud, you should use:
chunk.exportPoints("d:\test.ply", source = PhotoScan.DataSource.PointCloudData, format = PhotoScan.PointsFormatPLY)
By default the points will be exported in chunk coordinate system, if you need to use local coordinates, use projection = crs arguments in the export, where:crs = PhotoScan.CoordinateSystem('LOCAL_CS["Local Coordinates",LOCAL_DATUM["Local Datum",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]]]')