How to add interactive sorting for tablix and matrix header in the report
Find the following steps for adding the interactive sorting for tablix or matrix report item header
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Open the report in the report designer and right-click the text box in the column header that you want to add an interactive sort button, and then click Text Box Properties.

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Click User sort as shown in the following image.

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Provide the expressions in sort expression as shown in the following image.

You can repeat this action for multiple textboxes of the tablix or matrix headers.