Three-Tier vs. Multi-Tier

[Vanessa Hall]

For purposes of the Part 1 exam, is there a difference between a
three-tier architecture and a multi-tier architecture?
Section 2 of the objectives indicates that they are different, however
most of the articles and books that I have lump them together (as N-tier).
Can a three-tier architecture be horizontally scaled? i.e. if you
have a web-based system and decide to use a load balancer to employ
multiple web servers, is it still a three-tier system or do the extra
machines make it multi-tier?
Any help is greatly appreciated.
Thanks!

[Ajith Kallambella]

N-tier is a generalization( and inclusive ) of three-tier architecture and I prefer using the former than latter. N-tier systems often extend the standard three-tiers by including additional resources such as leagacy systems(EIS), service-based components(EAI) etc.
Tiers not necessarily are aligned with physical boundaries. For instance, you can have a single instance of an app server host both the web tier and the ejb tier. Tiers are conceptual architectural lines drawn mostly around the functionality provided by the components that make up the tier. Therefore, simply adding multiple web servers do not add additional tiers to the architecture.
Cheers,

[Hoan Vinh]

What if you have multiple client types making a request... i.e. a thin web client and a thick gui client.
Also, what if your business tier broadcast or publish messages to a topic or queue? Do you consider the MQ server a separate tier?