MapMovementCancelRequest

STOPS A WALK AND SAYS WHERE YOU ACTUALLY ARE. It is not "cancel my request" - by the time you send it you have already moved partway, and the one value it carries is where you came to rest.

WHEN TO SEND IT

WHEN A WALK YOU ALREADY STARTED IS INTERRUPTED BEFORE IT FINISHES. You send MapMovementRequest with a path, the server accepts it, and then something stops you early - the player clicked elsewhere, a fight started, an obstacle appeared, the player pressed the stop control. That is this message.

DO NOT SEND IT FOR A WALK THAT COMPLETED. A finished walk needs nothing; the server already knows the destination it approved.

SEND THE CELL YOU ARE ACTUALLY ON, NOT THE ONE YOU AIMED AT. This is the whole point of the message and the easiest thing to get wrong. The real client does not send the destination from the path it requested, nor the cell it last announced - it asks the moving entity's own physics for its current position and sends that. If you send the intended destination you are telling the server you completed a walk you interrupted, and your position and its position diverge from that moment on.

A script that walks by sending a path and then aborts must therefore track where it stopped. If you cannot compute that, do not send this message at all - diverging silently is worse than not cancelling.

WHAT COMES BACK

NOTHING DEDICATED. No dumped client waits for a reply. The server reconciles your position through the ordinary actor traffic that follows, so watch your own actor's position rather than expecting an acknowledgement.

BE HONEST ABOUT THE CELL and the correction will be invisible; be optimistic about it and the server will move you back, visibly, later.