Basically all content-types are automatically available, like
App.Data["content-type-name"]
For example, you could use it as follows:
var categories = CreateSource<ValueSort>(App.Data["Category"]);
or
// Sort by FullName
var sortedCats = CreateSource<ValueSort>(App.Data["Category"]);
sortedCats.Attributes = "Name";
Data.In.Add("Categories", sortedCats["Default"]);
or
// Just add the items which have the relationship to the category in the URL
var qsOfCat = CreateSource<RelationshipFilter>(App.Data["QandA"]);
qsOfCat.Relationship = "Categories";
qsOfCat.Filter = "[QueryString:Category]";
Data.In.Add("QandA", qsOfCat["Default"]);
or
public override void CustomizeData()
{
// new features in 6.1 - the App DataSource CreateSource<App> and also the RelationshipFilter
// Just add the items which have the relationship to the category in the URL
var qsOfCat = CreateSource<RelationshipFilter>(App.Data["QandA"]);
qsOfCat.Relationship = "Categories";
qsOfCat.Filter = "[QueryString:Category]";
Data.In.Add("QandA", qsOfCat["Default"]);
}