Hi Thanh,
The check points are points that are not used as GCPs. They are points with known coordinates that you mark on the images and at the quality report, you check the difference (computed position for the marked points - original position). You can have a look here: https://community.pix4d.com/t/7177-Using-checkpoints?page=1#community_comment_360002939371. Daniel has explained how to use checkpoints. Your workflow looks perfect! This video explains about checkpoints: https://youtu.be/qeJTbggYwik