Kirk, I wish to know your opinion -and everybody's too.
I used to talk about the "Presentation Layer", the "Business Layer" and so on. Later, while attending one Oracle's course (no ad intended) I learn some people prefer to say "Tier" instead of "Layer" when refering to the physical distribution of the components from a computing system.
Since it seems you don't use "Tier" but "Physical Layers", I am curious if you think the difference is not useful, or should we -as an industry- to create a common vocabulary.
I'm with Mohamed on this one. Tiers are where your code runs. Layers are how your code is structured. I talk about layers in the book. For instance, if you apply the physical layers patterns, you have the flexibility to run different layers across different tiers. And you gain this benefit because of modularity. It gives you greater flexibility.
Visit the book's website at modularity.kirkk.com where you can review all 18 patterns and download an excerpt of the book. There is also a mobile web application available that you can take with you wherever you go.