This is the configurability of the combobox control. multiSelect true means allow multiple selections, forceSelection true means to not allow the user to set the value of the field to arbitrary data. editable controls the user's ability to actually type in the field (which they could otherwise still do for the purposes of searching/type-ahead). For an alternative multiSelect implementation, you might also consider BoxSelect (which extends and uses the same config options as ComboBox, so the above comments still apply).