Hello,
We need to configure a value set provider that limits the content of the Categories; we will use this on both our Filed Against attribute as well as several other Category-based attributes.  We're pulling the info from a REST call to RTC.  While I can configure an HTTP Value Set Provider to return a set of results, you cannot use an HTTP Value Set Provider with a Category-based attribute (it doesn't appear as an option in the dropdown on the config).  You also cannot use Dependent Enumerations with them.  I have written a script to try and do it as you can select a script-based value set for Filed Against and other Category attributes.  I do not know if this will actually work as I do not know what value to return; would it be the name, id, itemId, or some other value that should come back?

dojo.provide("com.zions.valuesetprovider.categoryfilteralpha");
dojo.require("com.ibm.team.workitem.api.common.connectors.HttpConnectorParameters");

(function() {
    dojo.declare("com.zions.valuesetprovider.categoryfilteralpha", null, {
        getValueSet: function(attributeId, workItem, context){ 
            console.log("Going to do the parameters now");
            var params= new com.ibm.team.workitem.api.common.connectors.HttpConnectorParameters();
            params.url= "https://clm.cs.zionsbank.com/ccm/rpt/repository/workitem?fields=workitem/category[projectArea/itemId=_TofgQPfREeOpj9QcDboLoA]/*";
            params.xpath= "//workitem/category";
            params.columnXpaths= ["./id"];
            params.columnIds= ["id"];
            params.ignoreInvalidCertificates=true; 
            params.useOAuth=true;  
        var connector= context.getDataConnector("HttpConnector");
        var values= connector.get(params);
        console.log("length of returned values: " + values.length);
        /* none of my console logs ever show up for this script */
        var result= [];
        while(values.hasNext()){
            var entry= values.next();
            var getPushed= entry.getById("id");
            result.push(getPushed);
        }
        return result;
    }
});

})();

https://<clm uri here>/ccm/rpt/repository/workitem?fields=workitem/category[projectArea/itemId=<project id here>]/*