The Fixed Duration Attribute determines how fixed durations are assigned and stored. By default, Fixed Durations are stored in the Gantt Chart and can be manually set or updated using the Task Details panel or dragging its edge on the chart (when a fixed duration is set). By changing the Duration Attribute, you can automatically assign fixed durations to tasks, based on a Jira attribute or formula.
Format
If you use a custom field or formula for the Duration Attribute, you can also specify the format of values in that field.
This tells Gantt Charts how it should interpret the values in the field (milliseconds, minutes, hours, etc.). You can also select Text duration to use Jira duration format (1d 3h 30m).