Multi Select is a type of custom field that allows for multiple values to be selected at the same time. For example, a field named OperatingSystem may contain both entries Windows and Linux. Users can use this field in the More Filters dialog box to filter grids and on boards.
Select Admin > Project & Field Admin > Custom Fields.
In the appropriate asset section, and then select Add Multi Select from the Add Field drop-down button.
Select an asset from the RelatedAssetType drop-down list, and then click Save to save your changes and close the window.
Scroll all the way to the bottom of the Custom Fields page, and then click the Publish button. All unpublished fields will be immediately available within the system.