You can add row groups in the table to display data in a visual hierarchy. You can create both nested groups and adjacent groups. Row group can be added either using the cell context menu or using the group menu provided in grouping panel. In cell context menu the Row Group option will be shown only if there is a possibility to add row group in the respective place.
In tablix data region the data can be grouped using the following options:
The parent and child group are used for creating a hierarchy, the groups depend on each other. While the adjacent grouping functionality can be used to create separate groupings in the same table.
To add a first row group in the basic tablix structure follow the below steps.
Grouping Panel will be enabled in the design view.
To add first row group in the basic tablix structure, use the group menu in grouping panel.
Row Groups pane in grouping panel and open the context menu on the Details group field.

Parent Group... option under Add Group category.

Parent Group option, a Tablix Group dialog will be opened to configure the grouping.
Product Category field is chosen as parent group in tablix data region. Click on the OK button

Now, a new column will be added to the right side of the detail group in tablix data region and a new group member will be added above the Detail group of Row Groups pane in grouping panel hierarchical view.

On report preview, each product sales details will be grouped based on product category.

To sub categorize the data in tablix data region you can add child group to the tablix. Now, to add the child group we can either use the cell menu or group menu in grouping panel. In the following steps child group is added using cell menu.
Child Group under the Row Group category.

Child Group option, a Tablix Group dialog will be opened to configure the grouping.
Sub Category field is chosen as child group in tablix data region. Click on the OK button.

Now, a new column will be added to the right side of the Product Category row group in tablix data region and a new field will be added below of the Product Category group in hierarchical view of Row Groups pane in grouping panel.

On report preview, product names will be categorized based on the product type.

To add child group using group menu in grouping panel, refer Add child group section. Download the above report design from link.
To create multiple groupings in a tablix data region adjacent grouping can be used. The adjacent row group can be added above/before or below/after of the existing group in the data region. Now, to add the adjacent group we can either use the cell menu or group menu in grouping panel. Follow the below steps to add a adjacent group.

Adjacent Above to create a new group before the existing group or Adjacent Below to create a new group after the existing group.Adjacent Above or Adjacent Below option, a Tablix Group dialog will be opened to configure the grouping.

Group by field and click on the OK button.Now, a new row group will be created above or below the existing group in the tablix data region.
To add adjacent group using group menu in grouping panel, refer Add adjacent group section.
Row group can be deleted either using the cell context menu or using the group menu provided in grouping panel.

Delete Row Group option. Now, the following confirmation dialog will be launched.

Delete Group dialog choose one of the following options and click on the OK button.
Now, the respective group information will be deleted from the tablix data region.
To edit a row group properties in a tablix data region, refer Tablix member properties section.