在计算机软件领域，Layer与Tier都有”层”的意思，翻译为中文，也都是” 层”。但根据MSDN上的文章所述，这两个词的具体所指还是有
区别的.
一般认为,Layer指逻辑层，如我们通常据说的”三层架构”：UI(or Presentation) Layer, Business Logic Layer，Data Access Layer。
而tier指物理层。
下面是摘自Developer's Guide to EnterpriseLibrary 5 RC文档中第75页的一段话：
For simplicity, this example shows the principles of exception shielding at the level of the UI view.
The business functionality it uses may be in the same layer, in a separate business layer, or even on
a separate physical tier. Remember that you should design and implement an exception handling
strategy for individual layers or tiers in order to shield exceptions on the layer or service
boundaries.
下面是摘自EntLib50-combined(chm格式)文档中What Does the Exception Handling Application Block Do?
节：
This pattern ensures that your application does not leak sensitive information, no matter what run-time or system event may
occur to interrupt normal operation. And on a more granular level, it can prevent your assets from being revealed across layer,
tier, process, or service boundaries.
