The Row Level Security tab is used to enter additional UserID values for a user. These can then be used by the Row Level Security or Dynamic Pick List to substitute UserID value into the SQL to dynamically change the selection of data.
For example, the screenshot below for user Zane has 4 different UserID values entered. This might be from different systems throughout the company and you might need to use them for some reports that are using these systems. You might have a sales report that is using the accounting database and Zane's UserID in that system is ZSL and you want to use Row Level Security to filter the sales data on the report to show only sales from Zane. To accomplish this you can use the @UserID token when setting up the Row Level Security and it will dynamically change the Reports SQL statement to insert ZSL for the token. This will further filter the data from the report and will only display the sales data for Zane.
For more information on how to use the Row Level Security, see the Row Level Security section