how to mapping a single field of incoming SOAP to another message?

our scenario like this:e

the incoming message:

<header>

<interface>IF001</interface>

<description>xxyyzz</description>

<header>

<content>xxxxxxxxxxxxxxxxxxxxxxxx..........<content>

the field <content> is a xml string like:<?xml version="1.0" encoding="UTF-8"?><MT_Trg xm..........................

then we have to parse field <content> to different message according to field <interface>.

then i have 2 queations:

1. how to map a single field to a message

2. how determine target receiver message according to a conditon?

thanks in advance!