When multi select is enabled on a parameter in a SSRS report, this obviously allows the user running the report to make 1 or more selections from the parameter list. Whilst the selections appear in the report parameter drop down box after the selections have been made, it can be intuitive to add a label in the report showing the selections that were made for this parameter.
Create a Text Box in the report, create a Placeholder within the Text Box, highlight the Placeholder, right click and select the fx button next to the Value field. Enter the following expression to make your multi select parameter selections appear as a comma seperated list within the Text Box Placeholder on your report.
As an alternative you could optionally just display the word “Multiple” if there is more than one selection made.
=IIF(Parameters!ParameterName.Count = 1, JOIN(Parameters!ParameterName.Label, ", "), "Multiple")
You will need to substitute ParameterName with name of your Multi Select Parameter.