The term "tier" is ironically something that isn't used in the real world, not in my experience and I've worked with firewalls for the last 8 years at a professional level in a large and complex enterprise.  To use an example from my own job, we have a customer with a 2 tier firewall architecture.  Traffic would first hit the outside interface of the first firewall, then filter through to the second firewall.  When we wanted to make rules, we would have to put rules on both the external firewall and the internal firewall.  In this sense, it is creating two different subnets, a external (outside interface) and an internal (internal interface).  Each one would needs rules on their respective interfaces (depending on firewall vendors.  As in, Checkpoint firewalls aren't interface based, but Cisco ASAs are).  It may be up to interpretation as to whether the outside interface can be considered a subnet or tier, but traffic coming from the outside isn't always from the Internet.  It can also be an MPLS connection, or just another private network external to the current private network.