Following the question left on the previous blog entry Branching Out, I need to clear something up that may be a little confusing:

A Service Pack level and Service Pack installer are different things.

A client with Windows Vista installed that reports itself at Service Pack 2 has all of the hotfixes released prior to SP2 included in it.

The “Windows Vista Service Pack 2 installer”, however, contains only the hotfix packages since SP1.

 

(My previous blog entry was referring to how code changes are cumulative within hotfix packages on the same branch, and could be viewed as service pack level or installer equally as there was a change at each stage.
If changes “d” and “e” did not exist, then the SP1 installer would contain the binary with changes “a”, “b” and “c”, but the SP2 installer would not contain the binary at all – but installation media “at” SP2 level would contain the binary with all the 3 changes.)

 

Previously, Service Pack installers for Windows have been cumulative and so been very large – by the time the second Service Pack arrives it is almost guaranteed to contain files that are not necessary to update the vast majority of systems, and so is wasted space.

From Windows NT version 6.0, Microsoft returned to a single-servicing model – one source branch for Windows “Workstation” and “Server” products – where we had diverged from this from Windows 2000 (NT 5.0) as we went into XP (NT 5.1) and later Server 2003 (NT 5.2).

As you are aware, Windows Vista arrived before Server 2008 was completed, so the only “NT 6.0 RTM” product is “Windows Vista SP0”.
When Windows Server 2008 was ready for release it was forked off the NT 6.0 source branch and is the same product version, so it’s “RTM” level was “SP1”, and a Service Pack 1 installer was built for Windows Vista to bring it to the same level.

So hotfixes for Windows Vista prior to SP1 do not apply to Windows Server 2008 as they are already present.

Along comes “NT 6.0 Service Pack 2”, now with the collection of changes that were made since the fork at SP1, so it can be applied on Windows Server 2008 and Windows Vista SP1 equally to bring them to the same level.

 

More information can be found here:
Frequently Asked Questions: Windows Server 2008 Service Pack 2 and Windows Vista Service Pack 2