This statement should be interpreted as follows: the variable on the left-hand side of the statement is assigned the value of the expression on the right-hand side.
The expression and the variable must be of the same or compatible type. If the expression is of type STATE, ACTIVITY, etc., the variable is of the same type, or of type ELEMENT.
Here is an example of a template with a declaration section, followed by a section containing an assignment statement.
VARIABLE
LIST OF STATE st_list;
LIST OF ACTIVITY act_list;
LIST OF ELEMENT el_list;
BEGIN
.
.
el_list := st_list + act_list;
.
.
END;