ESIS communications

I am new to this whole integration phenomena. I have a decent understanding of the enterprise server functionality. My question is how is the document on the enterprise server transfered on to the integration server. Is ther a particular format/language that the webmthods integration server understands. Please help me clear up my confusion. Sorry if the questin appears too stupid.

In version 6, the webMethods Integration Server uses native communication with the Broker, just like the adapters did in the 4.6 world. You can think of the IS as an adapter if it helps. Once the document gets to the IS, it is treated like any other IS document - it’s stored in an internal format but can be easily converted to XML via built-in services.