ArUco grid or ChArUco board?
Both targets carry ArUco markers, and both give a camera something it can find. They are not interchangeable. Pick the wrong one and a calibration will be quietly worse than it should be.
What each one is
An ArUco grid is a rectangle of markers with an even gap between them. Every marker carries its own ID. OpenCV models it with cv2.aruco.GridBoard, which takes the marker length and one separation value.
A ChArUco board is a chessboard with an ArUco marker inside each white square. The markers say which corner is which, and the chessboard supplies the corners. OpenCV models it with cv2.aruco.CharucoBoard.
Why ChArUco wins for calibration
A marker corner is found where a black square meets white paper. The detector fits that corner from the marker outline, and the result is good to roughly a pixel.
A chessboard corner is different. Four regions meet at one point, two dark and two light, so the corner can be found by fitting the intensity around it. That reaches a fraction of a pixel. Camera calibration is a fit to those points, so better points give a better camera matrix.
The markers still earn their place. A plain chessboard has to be fully visible, and it cannot tell which corner is which if the board turns. The markers remove both problems, so a ChArUco board can be held at the edge of the frame and still contribute.
When a grid is the better choice
Use an ArUco grid when the target may be partly hidden, or when you want the pose of an object rather than the parameters of a camera. Any subset of the markers is enough to solve for the board pose, so a hand across one corner costs accuracy but not the answer.
A grid also suits markers that are cut apart and stuck on separate objects. Each marker keeps a known ID, and the sheet records which IDs went where.
A board that is not square
Give a ChArUco board a different number of squares across and down. A board with the same count both ways can be read at two different rotations, and nothing in the image says which one is right. Five by seven is a better shape than six by six.
One trap on older OpenCV
OpenCV changed the ChArUco layout at version 4.6. The old and new layouts differ only when the board has an even number of squares in both directions. A board printed to the current layout will not read on an older OpenCV, and the failure is silent.
Use OpenCV 4.6 or newer, or give the board an odd count in one direction, which makes both layouts agree. This generator says so on screen when both counts are even.