TurbofanDude...
I'm guessing you've never flown a real aeroplane, you know, ones that don't have these fancy screens.
In short, with an analogue flight deck, the ASI and ALT (both baro and radio) have one or more movable markers, the ASI having up to five, which can be selected to any position to mark that speed or altitude.
They are v/small plastic arrow shapes which slide on a raised 'rail' built into the outside edge of the face of the instrument. Each one has to be moved independently.
Just like the NG electronically marks the V1, Vr and V2 when you select confirmation on the FMC during preflight, (when the "No Vspeeds" indication disappears on the PFD), these markers have to be set manually.
These 'bugs' have no electronic input into any speed or altimeter system, they are purely there for the pilots' visual pleasure, a visual reminder.
Get the picture...
Cheers, EW73
