Multi select fields are available within a record, where more than one item can be applied to a field. 
Front Office
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To apply items to a multi select field, click in the field to see the available item list. Click on the item to apply. To add the next item where required, repeat the process.
Selections are removed by clicking on x.
Middle / Back Office
In the Middle / Back Office, multi-select options can display as a drop-down list.
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To select or de-select items, click in the checkbox to the left.