The selection method is an option for selection fields.
Multi Selection
Checkboxes - Users will see all the options and may select as many as apply.
Transfer to Selected List - Users will see a left and right box with an arrows in between. Users click on an option in the left-hand box and lick the arrow pointing to the right to move the selection to that box. Users may deselect by clicking the option in the right-hand box and clicking the arrow pointing to the left.