Class security Level Vs Instance Level Security Rights
You can configure the security rights either the class level or the instance level in SMS/SCCM.
These rights basically helpful in providing the access to the SMS objects like collection,packages,reports and many more too!
class security rights: It applies to the entire class like SMS site.Let say if you have collection called “All windows systems” and inturn it has child collection “All XP”,”All Vista” etc.If you apply class security rights on”All windows systems” it automatically applies to child collections.
Instance security rights:The name itself gives the answer,it is applied to an instance.If you want to provide rights only for “All Vista” no more to “all windows sysemts” and “all XP” .this can be helpful.
You can apply the same to provide access to one report using Instance Security.
these rights might be helpful in giving rights to the helpdesk in terms of either particular collection,reports or packages.
More information can be found here