Can someone tell me what this does? (see attached in red) It doesn't seem to corelate to anything.  The magenta box moves from F to S but why?  What does the F and S mean? 

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11. Fast/Slow Indicator
Displays airspeed information.
*indicates deviation from the airspeed selected by the FMC or the IAS/MACH selector, or limit speed
*small diamonds indicate 10 knots fast (F), or slow (S).

The above is taken from the ‘757 Captain III’ Manual Part II – Aircraft Systems (CS757CAPTAINIII_MANUAL_PART2.pdf) on page 230-231, and can also be found in ‘767 Captain II’ Manual Part II – Aircraft Systems (CS767CAPTAINII_MANUAL_PART2.pdf) on page 244. I am providing the info for both seeing as you don't mention which product the image is taken from.

Note. This info can also be found in the ‘757 Captain’  MANUAL Part II – Aircraft Systems (cs757_manual2.pdf) on page 51, and in the ‘767 Captain’ FLIGHT MANUAL Part II – Aircraft Systems  (cs767_manual2.pdf) on page 46.