A cause of frustration for many Festool MFT owners is the small amount of "slop" where the rail fits over the locating tab. While this play can be minimised by adjusting the tab so that there is side-pressure on the rail, the steel tab eventually wears into the soft aluminum and the problem only gets worse.
The "Slop-Stop" is a insert that solves this problem and eliminates the metal-to-metal contact. To use simply slide the insert in the MFT rail, centering it where the rail meets the tab (often the wear marks will be an obvious indicator). Then lower the rail, slide the "slop-stop" in either direction until the desired fit is acheived, then lightly tighten the set-screws with the supplied wrench. Finally check your rail alignment.
The "Slop-Stop" is machined from delrin and will provide excellent wear characteristics, but by reversing/rotating the insert, there are effectively four wear-surfaces, enough to last the normal user for years.