Volume Integral or Surface Integral for velocity
 

I have a question. I am going to get the velocity in all direction. So how can I get the velocity? through Surface Integral or through volume Integral?
And from either of them, the report type should be Mass weighted average or Area weighted average? Would you please give me some advice

usually dont difference between Surface Integral and volume Integral
usually i use Mass weighted average because in area average use your meshes and in bad meshing answer is not exat!

What does this mean?