We have been viewing the wiki page below to figure out how to do a particular customization.
https://jazz.net/wiki/bin/view/Main/AttributeCustomization

Our requirement is to add a Tag to a workitem called "clean" and after focus is lost clear the Description field of workitem. We copied the example under Script Based Value Set about setting the Severity enumeration, because it looked like we could also achieve what we wanted with this. Using this we could add the "clean" Tag to workitem and after losing focus it adjusts the contents of the Severity enumeration. Only problem is how can we add the Tag and clear the contents of the Description field. I don't think we can use array type for this field, which Value Sets must use. We couldn't figure out what other category we might use. How should we write our script? Here is roughly what we have:

dojo.provide("org.example.workitems.providers.SeverityTagValueSet");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
dojo.require("dojo.string");

(function() {
var WorkItemAttributes= com.ibm.team.workitem.api.common.WorkItemAttributes;

dojo.declare("org.example.workitems.providers.SeverityTagValueSet", null, {

    getValueSet: function(attributeId, workItem, configuration) {
   
        var tags= workItem.getValue(WorkItemAttributes.TAGS);

        var result= [];
       
        if (tags.indexOf("clear") != -1) {
            result.push("");
        }
       
        return result;
    }
});
})();

Here is the example we were copying:

dojo.provide("org.example.workitems.providers.SeverityTagValueSet");

dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
dojo.require("dojo.string");

(function() {
var WorkItemAttributes= com.ibm.team.workitem.api.common.WorkItemAttributes;

dojo.declare("org.example.workitems.providers.SeverityTagValueSet", null, {

    getValueSet: function(attributeId, workItem, configuration) {
    
        var tags= workItem.getValue(WorkItemAttributes.TAGS);
        var severityOptions= [];
        
        if (tags.indexOf("important") === -1) {
            severityOptions.push("severity.literal.l1"); // Unclassified
            severityOptions.push("severity.literal.l2"); // Minor
            severityOptions.push("severity.literal.l3"); // Normal
        }
        
        severityOptions.push("severity.literal.l4"); // Major
        severityOptions.push("severity.literal.l5"); // Critical
        severityOptions.push("severity.literal.l6"); // Blocker

        return severityOptions;
    }
});
})();