A user was seeing an "S" or a "B" in the lower portion of their viz when viewing their viz on Tableau Server.  When they hovered their mouse over it they got the following tooltip:

 

It turns out that this is due to the URL parameter "?:jsdebug=true" that was appended to the end of the user's URL and the answer came from Tableau's BJ Bjostad in his reply, Re: How to remove "Debug Mode" link at the bottom right of a report:

The embedded dashboard being viewed on the website has been embedded using the URL Parameter  "?:jsdebug=true". This parameter enables debug mode which displays whether the dashboard is being rendered locally (B) or on the server (S).

 

There is a list of URL parameters in the Tableau Server Administration Guide but I couldn't find this particular one.  I did a quick search in the other documents found in Tableau Help and didn't find anything for "jsdebug".  This makes me wonder what other undocumented URL parameters are available for us to use...

 

What would an administrator use this for?  Well, the first thing that comes to mind is the client-side rendering process when manipulating the Threshold Calculation.  This could be used to check where the session rendering is taking place when fine-tuning the Threshold Calculation.

 

Just remember to remove the URL parameter when you're done with it