MAPONLY means that no data from your program is to be merged into the map; only the information in the map is transmitted. We will use this option when we send the map the first time, because we will have no information to put into it.
DATAONLY is the logical opposite of MAPONLY. You use it to modify the variable data in a display that is already been created. Only the data from your program is sent to the screen. The constants in the map aren't sent; so you can logically use this option only after you have sent the same map without using the DATAONLY option.