Real-life example:
The company I worked for (broadband ISP):
Tier 1: Modem installation, Windows DUN troubleshooting, e-mail client support, login/pd changes, e-mail alias creation
Tier 1.5: DNS & MX records, account updates, static IP assignment, initial troubleshooting of ISP side routing problems, reporting of major problems to Tier 2
Tier 2: Major account issues, relatively minor routing problems
Tier 3: Major routing problems
Customers never actually talked to anyone past T1.5, & usually not even that. Tier 1 talked to 1.5 for the customer, 1.5 to 2, 2 to 3, etc.
Viper GTS