Does anyone knows the different between {searchTerms?} and {searchTerms}

Hi,

Some of the OOB search results sources use {searchTerms?}. Conversations is an example.

I wonder what is the question mark at the end of the variable?

The variable I know and use is {searchTerms}

The answer is missing from the following articles:

• Keyword Query Language (KQL) syntax reference http://msdn.microsoft.com/en-us/library/ee558911(v=office.15).aspx
• Query variables in SharePoint Server 2013 http://technet.microsoft.com/en-us/library/jj683123.aspx

Thank you,