The Class Type (and selected value) is more the container / location where it will be saved.
If you select a page type you could let it generate a basic transformation based on its own fields. But you are free to use any fields you like. Remember thought that since the transformation is part of the page type for deploying it you'll need to deploy the whole page type. So you could also create a blanco container page type just for this transformation.
I'm not sure of there are any limitations in creating your own custom transformations the free version though..
For the transformation itself I suppose you could just use the following:
<%# Eval("FieldName",true) %>