The "Assign To User" trigger allows you to trigger the assignment of a Contact to a User based on a condition of your choice.
Only Apply To Unassigned Contacts
The first thing to note is the "Only apply to unassigned contacts" toggle. If this is OFF and the trigger fires for a contact who has already been assigned to a User, the trigger will overwrite the previous assignment.
Single User Assignment
If you select just one User from the "Select User" dropdown, the Contact associated with the trigger condition will be assigned that specific User every time the trigger fires.
If you select more than one user from the Select User dropdown, the trigger will round-robin assign the contacts. For example: if you select two users (Tom and Jane); the first time the trigger fires it will assign the contact to Tom, the second time it fires it will assign the contact to Jane, the third time the trigger fires it will assign to the contact Tom, and so on and so on.
Weighted Round-Robin Assignment
When you select more than one user, you'll see that a dropdown option appears under the label "Split Traffic". By default, it is set to split the traffic equally as described in the example above. If you change the dropdown to "Unevenly", however, you will see several more fields appear, giving you the option to weight the distribution of the leads unevenly, according to the value you enter next to each person's name.
The value you enter indicates how many leads should be assigned to that user before moving on to the next user.
In the scenario of the image above, the 1st time the trigger fires, the contact would be assigned to Chase. The 2nd, 3rd, and 4th times the trigger fires, the leads would be assigned to David. The 5th - 9th times the trigger fires, the leads would be assigned to Ian, and then the 10th time the trigger fires, it would loop back around and the 10th contact would go to Chase again.