When SSRS exports the report as a CSV file, a CSV report has the following characteristics when rendered using the default settings:
Formatting and layout are ignored.
The following items are ignored during rendering:
Page header
Page footer
Custom report items
Line
Image
Rectangle
Automatic subtotals
The remaining report items are sorted, from top to bottom, then left to right.
For more details, you can check this link: https://learn.microsoft.com/en-us/sql/reporting-services/report-builder/exporting-to-a-csv-file-report-builder-and-ssrs?view=sql-server-ver16.
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