Action mustUnderstand header someone please explain it

I've been trying to find a simple explan开发者_StackOverflow社区ation on the web on this header.

What is its use? When do we need it? What does it do? What does it mean? and so on...

I know this is pretty simple but I've been looking for a well explained answer regarding this header online for more than an hour.

Action header describes what operation must be invoked on the service. MustUnderstand is general attribute for headers used in SOAP messages. Once header is marked with this attribute and either service or client doesn't have "handler" for that header (doesn't understand what to do with header) the processing should fail (should not must - for example intermediaries don't fail if they don't understand a header because the header doesn't have to be targeted to them). Once the Action header is used it is used together with MustUnderstand attribute.