Ground control points (GCPs) are used to georeference the model. A minimum of three GCPs is required to scale, rotate and locate the model, but we recommend that each project has between 5 and 10 GCPs distributed throughout the project area. If you have more GCPs in a project than necessary to accurately scale, rotate, and locate, some of the GCPs can be used as checkpoints to assess the accuracy of the project. GCPs improve the relative and the absolute accuracy of the model.
For more information about GCPs: Step 1. Before Starting a Project > 4. Getting GCPs on the field or through other sources (optional but recommended).
Checkpoints (CPs) are used to assess the absolute accuracy of the model. The marks of the checkpoints are used to estimate its 3D position as well as potential errors in the clicks. This way, the relative accuracy of the area of the checkpoints may improve. The difference between the initial and the computed positions of the checkpoints is displayed in the quality report and it gives an estimation of the absolute accuracy of the model in the region.
For more information about the quality control of a project: Quality report specifications.