The versatile string profile also gives you extra power.
The multifilament resin gives you better control over the shuttlecock.
The string gauge is the diameter of the string. The principle is pretty simple: a thicker string will be more durable, but will provide less feel. On the other hand, a thinner string will provide more feel but will also break more easily.
String gauges range from 0.75 mm (very durable) to 0.66 mm (very thin).
The BST800 string has a somewhat thick gauge of 0.70 mm.