When installing the Joomla multisite patches, one of the patches consists in restoring the installation directory.

This new Joomla multisite version just include the new Joomla installation directory 1.5.11.

No other changes in Joomla multisite functionality are present except its packaging with original joomla 1.5.11 files.

The update of Joomla 1.5.10 to 1.5.11 can also be done directly with previous joomla multisite version.
When we have experimented to joomla 1.5.11 with JMS 1.5.21 (previous one), joomla multisite has detect one patch to re-install due to a fix provided by the Joomla team.
The patch is applied to the Joomla 1.5.11 to benefit of the joomla fix. Even with previous Joomla MultiSite versions.