Get Latest Maintenance downloads the latest maintenance for the product/release for which it was performed from the CA sites ( ftp.ca.com or sdownloads.ca.com) into your CA CSM software catalog. By software catalog I mean into the CA CSM controlled USS directories.
These are actual PTF's that replace modules. MAC's SRC' etc. This does not involve applying these PTF's, they are just downloaded into the software catalog.
Before I explain Upgrade CA RS Level , I think you might be aware of this but just in case, CA RS are ++ASSIGN statements that associates PTF's to a CA RS sourceid such was CAR1706 for example.
CA RS can be downloaded automatically from from system settings, software catalog and also by clicking the update immediately button.
When an Upgrade CA RS Level is done CA CSM will use the latest CA RS sourceid in the software catalog, look at each ++ASSIGN statement for the level such as CAR1706 to see what PTF's ( the ones downloaded with get latest maintenance ) are in that CAR1706 and that are related to the product/release for which the Upgrade CA RS Level is being done and apply those PTF's to the CSI for that product/release.
Also note that CA CSM applies CA RS Levels cumulatively, meaning it will automatically apply from the latest to other previous levels that have not yet been applied for the product/release.