What is "Lookup" in source drop down in data sources? This option comes when selects page in structure drop down. In developer help i don't see comprehensive description in lookup as data source option. Please let us know more info about this.
As you mentioned the "Lookup" option is available to populate a data page as long as you choose "Page" as the Data Page structure. When using this you are populating the data page with an instance of a class. So you will configure this by inputting the class name and a parameter to identify which instance to use at runtime. This is very handy when you need to update the data page with an entire page of data that is conveniently found in an existing instance of a class.