You can use default values in a search mask to:

• pre-fill part of a standard field
• set a hidden default to always filter the results on a value

A common use of pre-fill is for a document ID search where the prefix is usually the same. For example, if you have a managed property DocId configured correctly:

<Control Name="DocId" Type="Text" DisplayName="Document ID" Value="DB-" />

An example of a hidden default is in our standard Email search mask. This line ensures results are always msg or eml files: 

<Control Name="FileExtension" Type="Hidden" Value="msg;eml" />