Can't find good documents on WIX XmlConfig. There are some posts with examples that help me getting started:

• http://blogs.msdn.com/gisenberg/archive/2007/10/09/wix-v3-and-xmlconfig-xmlfile-troubleshooting.aspx
• http://www.mail-archive.com/wix-users@lists.sourceforge.net/msg18757.html

Here is the summary of what I learned from the try-and-error approach,  

VerifyPath

• when setting attribute value, it has no effect
• when creating element
• if exists, XmlConfig does nothing - skip the action and also all the XmlConfig that reference the id;
• if not, creates the element, and other XmlConfig that reference the Id;

ElementPath reference Id

• Does not work when Node is 'value' - it fails saying the path can not be found.
• It works when Node is 'element'

Modifying attribute:

    <util:XmlConfig Id='SetServiceCertificateFindValue'
                    On='install'
                    Action='create'
                    Sequence='100'
                    File='[WebConfigFilePath]'
                    ElementPath='[XPath_serviceCredentials]/serviceCertificate'
                    Node='value'
                    Name='findValue'
                    Value='[SERVICE_CERTIFICATE_FIND_VALUE]' />