When writing a project method, there are a couple ways to assign the value of its output:

a) Assigning a value to the declared return parameter

#DECLARE($input : Text)->$output : Text $msg:=$input+"foobar" $output:=$msg

b) Using a return statement

#DECLARE($input : Text) : Text $msg:=$input+"foobar" return $msg

Both examples above function similarly in outputting the same value, though (a) has the added benefit of type checking. Note that (b) stops execution of the method, while (a) does not necessarily. If both were to be used in the same method, (b) would always take precedence over (a) to produce the output.