The Details group can be used to display the detailed data in the tablix data region and it has no group expression. By default, when you drag and drop the the tablix report item in the design area,it renders with a Details group.
Grouping Panel will be enabled in the design view.

Row Groups pane in grouping panel and open the context menu on the innermost child group. Click Add Group, and then click on the Child Group option.

Child Group option, a Tablix Group dialog will be opened to configure the grouping.

Show detail data and click on the OK button.

Cannot insert a multi-value item into a detail cell. Detail members can only contain the static inner members.
Details group can be deleted either using the cell context menu or using the group menu provided in grouping panel.
Details group member. Click on Delete Group option in the menu.


Choose the Delete group and related rows and columns option to delete all the rows and columns associated with the respective group.
Choose the Delete group only option to delete the group alone.
Now, the respective Details group information will be deleted from the tablix data region.
A details group is added as a child group in the Grouping panel, so you cannot add child group to the details group further.
To edit a group properties in a tablix data region, refer Tablix member properties section.