Hello Craig,
In normal repository update operation, the entire repository is updated to the latest version. This leads to the downtime of all the SnapManager for Oracle/SAP servers and mandatory that all the server should be upgraded with the latest version of SnapManager.
To avoid this downtime of all the servers using a repository, rolling upgrade facilitates upgrading specified hosts in the repository to use with newer version of SnapManager, i.e. you will able to upgrade the selected hosts without affecting the non upgraded hosts.
And during the rolling upgrade process, tables pertaining to new smo version will be created in the existing repository, and all the required host data ( related to hosts being upgraded) will be deleted from old tables after copying it to newly created tables. Hosts which are not upgraded will still have their repository data in the tables created for older version in the repository.
I will also recommend to look at the rolling upgrade considerations listed in the installation and administration guide for SMO.
Thanks,
Himanshu