...
When creating or editing a custom field with an Input Type of List, users may now choose to make it multi-select! Simply click the Yes radio button in the Multi-Select field. When enabled, users completing the custom field can select multiple options from the list. Additionally, when Multi-Select is enabled, use the Custom Fields import (Import > Custom Fields) to include more than one value by adding a colon between each value in the column (e.g., one:two:three).
Resources:
Cost Center Drop-downs moving to Search Quick Filters
...