Field Help, Input Help, and Dropdown Boxes 
Field help (F1) and input help (F4) are standard functions throughout the system. You therefore cannot assign any other function codes to the F1 or F4 keys.
), a help text appears for the field in which the cursor is currently positioned.
) to the right of a screen field, a list of possible entries appears for the cursor in which the cursor is currently positioned. The user can then choose one or more values, which are then copied into the screen field. There are various ways of making field and input help available to users. For example, you can either use ABAP Dictionary functions, or program your own help functions.
Another special way to display lists of values is to use a dropdown box.
Field Help Input Help Dropdown Boxes