Attention using Context Vector: Hierarchical Attention Networks for Document Classification
I can not completely agree with this previous answer.

The context vector $u_w$ is not computed by the Eq. 5 and 6. 
As stated in the paper, $u_w$ is randomly initialised and learned during the training. 
Instead, it is $u_{it}$ that is computed by Eq. 5 and 6, using the (again) random initialised  $W_w$ and $b_w$.