An informal definition for a tier-2 system is one that helps you to operate better. A tier-1 system helps you operate, meaning that without the system being up, you cannot function. For example: Eventlogging data is used to help us improve our applications but we can certainly function without it.
Being tier-2 means that we provide support for the service/application during business hours in the absence of any tier-1 issue that might be affecting our infrastructure. Any of our services and applications could go down and be down for 48 hours (a weekend), so you should be sure your reporting can deal with gaps in the data.