It defines some semantics for how to treat headers such as whether processing of the header is optional.
它定义了一些关于如何处理消息头的语义，比如头的处理是不是可选的。
2
Like a cascading style sheet, this type of processing instruction is optional, even for a transformation tool.
与级联样式表相似，这种类型的处理指令是可选的，甚至对于转换工具来说也是这样。
3
Doing this enables us to use a mediation to apply optional messaging processing, such as filtering the message content or routing to a different destination, just to name a few.