The trailing edge flap position indicating system provides flap position and flap system failure information to the flight deck.
Flap position is indicated by a dial indicator with needles for indicating right and left wing flap and slat position.
The flap position indicator needles are normally driven by the outboard synchro position transmitters in each wing.
The primary source of flap and slat position information is the flap/slat position indicator located on the P3 panel.
The position indicator uses two needles, "L" and "R" to indicate flap and slat position between the 1-unit and the 30-unit positions.
The position indicator shows operation of the slats between the UP and the takeoff position.
When any slat leaves the retracted position the position indicator needles move halfway to the 1-unit mark.
When the slats reach the takeoff position the needles move to the 1-unit position.
Here is SSM reference: