When developing a custom Expiration formula in SharePoint you may want to pass in parameters/config data for the formula, e.g. only expire items where field X has value Y.

The method that you needs to implement for your formula has the below signature:

public DateTime? ComputeExpireDate(SPListItem item, XmlNode parametersData)
{ }

You can pass params/config data to your formula via the parametersData parameter. This is a piece of XML that will have the following schema:

<formula id="[the ID you assigned to your custom formula]">
    <!--Your custom XML, e.g. the below -->
    <documentStatusFieldName>Status</documentStatusFieldName>
    <statusValueToExpire>Archived</statusValueToExpire>
</formula>

You can set the XML contained within parametersData when you enable the Expiration policy feature for a policy using code. The code below for example enables the Expiration policy feature for a content type. The code configures the Expiration policy feature to use a custom Expiration formula, and pass in XML to be used inside that formula (via parametersData).

//Needs using Microsoft.Office.RecordsManagement.InformationPolicy;

public void EnableExpirationForContentType(SPContentType contentType)
{
            const string EXPIRATION_POLICY_FEATURE_ID = "Microsoft.Office.RecordsManagement.PolicyFeatures.Expiration";

            //Ensure that a policy is defined for the content type
            Policy policy = Policy.GetPolicy(contentType);
            if (policy == null)
            {
                Policy.CreatePolicy(contentType, null);
                policy = Policy.GetPolicy(contentType);
            }

            //Always overwrite any existing Expiration setting for the policy (i.e. delete if exists)
            if (policy.Items[EXPIRATION_POLICY_FEATURE_ID] != null)
            {
                policy.Items.Delete(EXPIRATION_POLICY_FEATURE_ID);
            }

            //The id attribute must points to an Expiration formula you have deployed to the server
            string policyFeatureXml =
@”<data>
    <formula id=”"MyCustomExpirationFormula”">
        <!—From here on is your custom XML—>
        <documentStatusFieldName>DocumentStatus</documentStatusFieldName>
        <documentStatusToExpire>Archived</documentStatusToExpire>
    </formula>
    <action type="workflow" id="[guid of a workflow that has been associated with the content type]" />
</data>";

            //Enable the Expiration policy feature
            policy.Items.Add(EXPIRATION_POLICY_FEATURE_ID, policyFeatureXml);
}

Note the id attribute in the XML must point to an Expiration formula you have deployed to the server. Follow this tutorial for how to develop, deploy and debug a custom Expiration formula: http://blah.winsmarts.com/2008-10-Authoring_custom_expiration_policies_and_actions_in_SharePoint_2007.aspx.