Service or group of services that must start before the parent service.
Windows Installer references
Group (YesNoTypeUnion) : Set to 'yes' to indicate that the value in the Id attribute is the name of a group of services. If 'no' or not set, the Id attribute refers to another service by name.
Id (String, required) : The name (not the display name) of a previously installed service or the name of a service group when the Group attribute is set to 'yes'.