What's the difference between EXCHANGE(*NOW) and EXCHANGE(*ALWAYS)?

*NOW, which is the default value, indicates that all nominated fields should be mapped into the exchange list now (ie: at this point in the program logic). This version of the EXCHANGE command is considered to be an executable command because it causes the fields to be placed into the exchange list at this point in the RDML program (and nowhere else).

*ALWAYS, the other allowable value, indicates that all nominated fields should be automatically mapped into the exchange list at several pre-defined points during the program's execution. These pre-defined points are:
• during any type of normal program termination
• before calling a process or function (CALL command)
• before invoking a PROMPT_KEY handling process or function
• before submitting a process or function (SUBMIT command