Normally, when labelers press on Start Labeling, they are shown the first task that is available to them on the labeling queue. And, given that the labeling queue is normally shared between all labelers in a project, this results in labelers picking one task each from a common pool.
In some occasions, however, it might be necessary to assign certain labeling tasks to certain labelers, or reviewing tasks to certain reviewers, so that the task cannot be completed by someone else. This is where Task Assignment comes into play.
Only tasks with a status of "To-Do" which have not already been assigned can be assigned.
Optionally, using the filter controls at the top of the list, narrow down the task list so that you only see the tasks you wish to assign to the labeler or reviewer.
Once the tasks are into view, select them by clicking on their checkboxes (highlighted in red on the screenshot):
Clicking on the box highlighted in green on the screenshot above allows you to select all tasks currently in view.
Note: when selecting all tasks this way, only the tasks currently in view are selected. Tasks in the list but not in the current view/page are not selected.
From the Item Actions dropdown, navigate to the Assign To action and click on it. The following dialog will appear:
By picking an option between Assign as Labeler and Assign as Reviewer, you may choose whether you'd like to assign a certain task to a user for labeling or reviewing.
From the Select from Labelers/Reviewers dropdown, you'll be able to pick the person to which the task will be assigned for labeling or reviewing.
If a task had already been assigned to someone else, it cannot be reassigned. It first needs to be re-queued by removing the labeler/reviewer. Only then it can be assigned again.