The Windows Installer will replace the syntax [PropertyId] with the Property's value at install time for certain fields. The MSI and WiX SDKs will indicate which fields support "formatted data" (i.e. Property replacements). If a field is not "formatted data" the [PropertyId] expression will not be evaluated and the value will be left as [PropertyId] (rarely, the intended result).

For example, the following puts the value of the Property with Id='FOO' in a registry value:

<Component Directory='InstallFolder'>
  <RegistryValue Root='HKLM' Key='SOFTWARE\Manufacturer\Product'
    Name='Foo' Value='[FOO]' Type='string' />
</Component>

The formatted syntax also supports a few prefixes to the identifier to get values from items other than Properties. The following is a short list of the most useful:

• [!#FileId] - evaluates to the full path, including file name, where the File with Id='FileId' will be installed.
• [$ComponentId] - evaluates to the full path of the folder where the Component with Id='ComponentId' will be installed.

Note: the paths are only available after CostFinalize action has completed.

A full list of the available formatting can be found in the MSI SDK's Formatted topic.