What does “wcm:action” denote in unattend.xml?

When configuring an answer file for WinPE, there are various elements that take a wcm:action attribute with the value add. For example, RunSynchronousCommand, NetBT Interfaces, etc., all have examples roughly like the following:

<RunSynchronous>
   <RunSynchronousCommand wcm:action="add">

However, I cannot find any documentation at all of what this attribute actually does. Can it take values other than add? What is the consequence of leaving it out? (It appears to make no difference in my testing.)

Over on ServerFault several attempts were made to locate or generate a schema for the answer file. However, as one comment notes, the schema for the wcm namespace (http://schemas.microsoft.com/WMIConfig/2002/State) does not appear to be present.