setting of UDS
 

If I don't need to set boundary values in some places, do I just keep the default setting which is "Specified Flux equals zero"?

http://cdn.img.headns.com/2011/0948828f2cc94.png

fast answer is yes.

however, the real answer comes from concept of the model you are going to simulate. Actually, you should know from the very begging what are boundary conditions in your model