To use multi select on an issue, follow the steps below:
Before using the database custom field of the multi select list type, ensure that the type of the custom field is "Select List" and the Multiple Values checkbox is selected in the field configuration age, as shown below:
The dbi custom field will now display values in a combo box in the create, edit, or transition screen of an issue, enabling you to select multiple values using Ctrl + click.
After you save the ticket, multiple values selected for the custom field are displayed in the view issue screen, separated by comma.
To specify whether to show the "None" option in the multi select field, select the Add None Option checkbox for this custom field on the Administration page.