To be clear it doesn’t guarantee that the value was updated in TM1 but it will return an error if it failed. Exiting a writable cell sends a request to TM1 to update the value if it has changed and returns the result.
Try typing a string into a numeric cell and you will see an example of an error being returned.
Thanks. It sounds like a Canvas DBR functions very much like a DBRW or DBS in Excel i.e. the value you see in the cell is the “echo” of the cell value’s round trip to the TM1 Server.