As you sink your teeth into the depths of BizTalk 2006 R2/2009 and realise the potential
of integrating with the WCF Custom Adapter, a new world opens up.

Then

Then at one point down the track you’ll want to tweak or customise what is being sent/received.
For e.g. changing the shape of the XML transferred over a HTTP transports – maybe
compression, maybe minus the explicit XML notation (i.e. takes 5KB to send over 30
chars)

So you’ll then move onto something called a WCF Custom Behaviour.

Two great whitepapers are

Part 1- http://msdn.microsoft.com/en-us/library/cc952299.aspx

Part 2- http://msdn.microsoft.com/en-us/library/dd379134.aspx