Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Available in Sandboxed Solutions: Yes
Available in SharePoint Online
If the SPWebTemplate represents a site definition configuration, this property is a concatenation of the Name attribute of a Template element in a WebTemp*.xml file and the ID attribute of one of the child Configuration elements of the Template element. (See Understanding WebTemp*.xml Files for more information.) There is a "#" character in between the two parts; for example "MPS#1" or "MyCustomSiteDefinition#0".