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Everything outside the direct XP updates like .NET, Silverlight, MSSE ect. However, sooner or later those will fail to install on XP! You are completely on your own.

these 'outside' updates provided by wsusou after April 2014 is a subset of 'outside' updates offered by the service 'Automatic updates' of XP?
(I say subset because of the limitation given at viewtopic.php?f=7&t=172 :
"This catalog file contains at least all the updates classified as "critical" and "security relevant", but it does not necessarily contain all "important" and "optional" ones.")

in other words, there will be no update offered by wsusou and not offered by the service 'Automatic updates'?

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"those will fail to install" ... you mean one day some updates needn't to run on XP?

wsusou will install the security relevant updates, Windows Update would install too. It just won't install the optional ones.
But some updates (I'm talking about .NET and updates like this) may require a newer version of Windows and due to this may fail to install / cause problems, when installed.

Thank you for all the answers.