Somewhat atypical, the life cycle of this Mac OS X 10.6.4: it was officially released to everyone just over two months ago, but in late July it had its Server version updated by Apple. A few days later he got another update for the new iMacs, and now the same is true for Macs Pro.
On the support page about Mac OS X 10.6.4 Update Mac Pro (Mid 2010) Apple does not detail news regarding the original version of Mac OS X 10.6.4, but states that it is recommended for all servers running Snow Leopard Server 10.6.3.
The new update weighs 32.7MB. More information can be obtained in this online documentation.