Documents in Web Validation can be assigned to users in 3 ways.
Example:
If you have configured 3 users to have access to a particular Web Validation step and have 9 documents that come into the Web Validation step, the documents get distributed to the users like this:

Note: Each user can only see the documents assigned to them in Web Validation. For example the blue user on the left will only see three documents (1, 4 and 7).
In the Process Monitor, you can see to whom the document is assigned to.

In the above screenshot, the column 'User' shows to which user a document has been assigned.
Documents can be manually re-assigned in the Process Monitor.
To do this, select the document to be assigned to a different user (multiple documetns or all documents can be selected)
Click on 'Assign to...'
Choose the user to assign the document to

Example: If you have configured 3 users to have access to a particular Web Validation step and have 9 documents that come into the Web Validation step, the documents are visible to all users in Web Validation:

In Web Validation, the screens will display all documents, and an indication shows which documents are 'in use' by other users.

The screenshot above shows two Web Validation screens with two different users logged in. The user on the left logged in first and is working on the first document. The right user logged in afterwards and sees that the first document is 'in use'. The second document is automatically selected.
If the user wants to see who is working on the other document and when the user started working on it, the user can hover with the mouse over that document and a tool tip will indicate the information.

When a user completes a document (presses the complete button), the other users will see a 'completed' indication and then the document disappears from their screen.

The setting if Round Robin or Pool is used can be set on the user task step in the workflow.
Click on settings for the particular user task (Web Validation)
Then switch between Round Robin and Pool by switch on/off the 'Assign users Round Robin'. On means the user is assigned in round robin, off means it is a pool.

The process flow needs to have two steps.
1. A ScriptTask before the Web Validation step
2. The Web Validation step:

In the above flow, the assigning of the user happens in the Script Task 'Assign to Scan Operator'.
The script to assign the user looks like this:
_userId = 10032;
In the above script, the user is assigned to a string value. However, you will need to ensure that this user exists in the Authentication - Accounts. Also, you need to ensure the user has access rights to the Web Validation step. Otherwise the user will be unable to process the document.
Sometimes it is useful to assign a document back to the scan operator (or mobile user). This could be important if the scan operator needs to make an update to the document or verify against the original (paper) document if a document is correctly scanned. The following script does assign the Web Validation user to the scan user:
_userId = _authAccountId;
the _userId is the Web Validation user and the _authAccountId is the user that submitted the document. The origin of the document can be from a Scan Client (Scan+ProcessLite, Scan+Express, Mobile Capture etc.).
In case a user ID should be reset (means not assign any user) user the following code:
_userId = null;