HP OV Operation Performance Agent

I’ve been looking into HP OpenView brocher and found out that they have coded several tier for the System above (subject title).
My questions is – What is “Tier” mean? How they defind the “Tier” for the specified server (Teir 0, Tier 1, Tier 2, etc)?
I’ve asked HP technical person but they fail to elaborate more on this matter. Could someone please help me?..
Thanks.