Meaning of SERVERPROPERTY ('ResourceLastUpdateDateTime') I have researched a while, a lot of articles name it and tell us to use it - but none really explains the meaning of the date and WHEN exactly it changes. Does anyone know? 

Samples for articles not really explaining the meaning of this property: 


https://msdn.microsoft.com/en-us/library/ms190940.aspx
http://www.mytechmantra.com/LearnSQLServer/How-to-Identify-the-Location-of-Resource-Database-in-SQL-Server/


Edit: The backstory behind the question simply does not exist. I just want to know what event forces this information to get a new timestamp. What makes the resource db getting updated?
Based on the BOL article you referenced to, the date is not going to change unless a Service Pack or CU creates new system objects. However the date returned may not necessarily coincide with the date you install that CU or SP.


  The Resource database makes upgrading to a new version of SQL Server
  an easier and faster procedure. In earlier versions of SQL Server,
  upgrading required dropping and creating system objects. Because the
  Resource database file contains all system objects, an upgrade is now
  accomplished simply by copying the single Resource database file to
  the local server.


The only time you will notably see that value change is when you apply a service pack or upgrade SQL Server. It will likely change the same time you see the ResourceVersion value change. Which if I recall I only see the version number change with service packs.

I have an instance of SQL Server 2008 R2 that shows a build of 10.50.6220, but the resource database shows 10.50.6000. Which the date returned is 08/09/2014 13:40:43, that is not when I applied the SP but probably the time frame that Microsoft's SQL Server Team finalized their testing. SP3 was released 09/26/2014.