Creating a Slice
To create a new slice, open the Gantt chart configuration, choose Slices from the left menu, and click the New slice button.

Slice Name
All new slices are named "New Slice" by default. To give your slice a custom name, simply replace this name.

Assigning Work Items
You can assign work items to a slice by selecting specific work item types or with a JQL query.

When creating your chart, Gantt will search for the items you specified and apply the custom settings you configure in your slice, before applying your default settings to the rest of your work items.
JQL Limitations
JQL queries should be based on work item properties.
Queries that rely on time comparisons, time-based functions – now(), endOfDay(), startOfDay(), endOfMonth(), etc. – or other variables that could change independent of work item changes, may not work properly with slices, and should be avoided.