The Defined difference is Level 20 security sign-on only and Level 30 security Sign-on as well as resource security. The main difference between level 20 and 30 is that at level 30 users are not automatically granted access to all resources in the system or the *ALLOBJ authority. If every user profile on the system already has *allobj authority then you have a serious security risk to the system and should consider using Group Profiles Or authorizations list to enforce a well balance security policy. If you chose to run with *allobj allowed you better make sure you keep good backups.
Here is a resource for System security Level
Here some info on Authorization list (require registration to view)