BCR402R - what is the purpose of Rext in this LED driver?
It allows you to increase the current. 
To do so, you need to know the value of the internal R : it's about 0.6(V)/0.01(I) = 60 ohms. Rext=60 ohms would double the current to 20ma.

Leave the pin open; connecting it to Vs is the same as asking for infinite current; not a good idea...