The Tableau Data Customization file, or .tdc, will automatically be called when a connection is made to a type of data source. It over-rides the default settings that Tableau and the data source may have. Note that the .tdc does not specify a particular data source nor instance (PRODDB, QA, etc.), just the type of connection. In this case it’s any MS SQL Server connection.
The attached Word document and .tdc file are what I send to my end users when they get a Desktop license and will be connecting to our MS SQL Server data source. It forces the MS SQL Server isolation level to the less strict "Read Uncommitted" since the database default is more strict. This .tdc file is also saved in the appropriate location on the Tableau Servers. This insures long extracts won't disrupt production and data warehousing processes.
Please see http://kb.tableau.com/articles/knowledgebase/query-isolation-levels?lang=en-us for additional reading for those that need to enforce their isolation levels.
Customizing and Tuning ODBC Connections | Tableau Software. Note that the VERSION variable is not defining the data source version but the highest version of Tableau Desktop the .TDC will be applied to during connection to the specified data source. For example, a .TDC file saved with the following <connection-customization> variables
<connection-customization class='oracle' enabled='true' version='9.5'>
will only be applied to Oracle connections in Tableau Desktop version 9 or lower. If an Oracle connection is established in version 10, the .TDC file will not be applied to the data source! This means the version variable must be updated every time Tableau Desktop is upgraded. Same situation applies if the file is used for Tableau Server.