The editor for a Map is broken into two main windows - the Main Screen and Designer.
The Main Screen is the initial window that opens when creating or editing a Map. This screen allows you to input the main parameters of the Map. These should be populated prior to opening the Designer window.
When editing a Map that configuration will not be saved unless you click the Save button on the Main Screen. There is also a Save Map button on the Designer to allow you to save your configuration to disk without leaving the Map.
The Designer window allows you to define the script code of your map in relation to its source and destination data definitions. This script code is the logic that provides the transformation of data from the source to destination data definitions. The Designer window is accessed by clicking the Design button on the "Main Screen".
The Designer opens as a separate modal window, this means you are unable to use other windows of Flow until you close this window. If you minimise this Window you will find that you cannot use the other windows of Flow.
When changing the source or destination data definitions on a Map, it will ask whether you wish to clear the current script code since the source or destination data definitions have changed. You will generally want to clear the mapping however sometimes it is useful to retain the old script code as an indication to help you complete the Map.