The Tail Length Setting refers to the length of thread that hangs out of the Grabber Trap. There is a setting to change the length of this tail to: Short, Medium and Long. There are some application specific situations where changing this setting is required, an example is when sewing with Metallic Thread.
Changing Tail Length:
To change the Tail Length go to Tools > Settings > Machines Tab and you can change the Tail Length with a drop down list. After selecting the Tail Length click Apply and OK.