The fields in this action are calculation fields.
So they contain an expression, such as:
1 * Variable + 3 * FUNCTION( 4 * Variable )
SCREEN is a function, and its first parameter can be one of a number of special tokens, including any of:
Main, Second, Third, Internal, External, Front, Back, Back2, Mouse
If it starts with none of them, then it is evaluated as an expression, so
SCREEN( Variable, Width )
is fine. Remember though, if Variable does not currently have a numeric value, then the expression will not evaluate and will turn red.
If you then set the variable to a numeric value (eg in the Preferences, Variable pane, or by Trying and action or running the macro), then the expression will now be valid.
