Various uses of Assign Message Policy  

AssignMessage is an extremely diverse policy using which you can do a wide variety of things. There is good amount of descriptions on the AssignMessage documentation page but I am adding a few more here:

Set Up URL Redirections

<Set>
        <Headers>
            <Header name="Location">{config.protocol}://{config.organization}-{config.environment}.{config.domain}/web?access_token={access_token}</Header>
        </Headers>
        <StatusCode>302</StatusCode>
        <ReasonPhrase>Redirect</ReasonPhrase>
    </Set>

Setup multiple variables

This is needed if you want to setup a number of global variables at once to be referenced by other policies later in the pipeline.

    <AssignVariable>
        <Name>appSecret</Name>
        <Value>abcdedl1234</Value>
    </AssignVariable>
    <AssignVariable>
        <Name>config.environment</Name>
        <Value>test</Value>
    </AssignVariable>
    <AssignVariable>
        <Name>config.protocol</Name>
        <Value>https</Value>
    </AssignVariable>

CORS enable your APIs

Below is for CORS-Preflight

<Set>
     <Headers>
        <Header name="Access-Control-Allow-Origin">*</Header>
        <Header name="Access-Control-Allow-Methods">POST, GET, OPTIONS</Header>
        <Header name="Access-Control-Allow-Headers">Accept, Content-Type, Authorization, x-requested-with</Header>
        <Header name="Access-Control-Allow-Credentials">true</Header>
     </Headers>
</Set>

Setup a target url different than what is setup at target.xml.

Or also known as dynamic target. If based on a condition you need to manipulate the URL. You can do the same using javascript as well and javascript often adds more flexibility , but if the requirements are simple AssignMessage can do the trick as well.

<AssignVariable>
        <Name>target.url</Name>
        <Value>https://api.usergrid.com/orgName/sandbox/products/{myretail.productid}/has/reviews</Value>
    </AssignVariable>

Setting custom error messages

<Set>
        <Payload contentType="application/json">{"error": {"message":"Not Found", "detail":"The requested resource was not found."}}</Payload>
        <StatusCode>404</StatusCode>
        <ReasonPhrase>Not Found</ReasonPhrase>
    </Set>

If anyone has any other interesting use of the AssignMessage policy please add them in the comments.