我是个刚接触数控的新手，自己在看840D的FB1，下面是其中的一段程序：
CALL FC 22( //Tool management direction selection
MagNo := 2, //Magazine number
ReqPos := fw 20, //Destination position
ActPos := fw 22, //Current position
Offset := b#16#0, //Offset for special positioning
Start := m 30.4, //Activate start
//Return parameter
Cw := f 30.0, //Move magazine in clockwise
//direction
Ccw := f 30.1, //Move magazine in counterclock-
//wise direction
InPos := f 30.2, //Magazine in position
Diff := fw 32, //Differential path
Error := f 30.3 //Error has occurred
);
FC22调用的一个例子，其中的f和fw是什么意思呀，S7-300中好象没有这个标识呀。