If you want to save only a specific field on your entity instead of saving the whole entity you can. In a submodule of the fields module you ll find a module called field_sql_storage module. In that module all functions available for the sql storage backend can be used. One of them is field_sql_storage_field_storage_write, which you give your entity, type, operator (update, insert) and which fields to save.
This project allows you to have conditional fields.
Example : we have a content type bus and tram with only two fields difference. The type gearbox for a bus and the type of controller for a tram. All the 10 other fields are just the same. You dont want a new content type for that. You need a conditional field.
So create content type vehicle and have vehicle type field, now set the other fields like gearbox - controller conditional to this one. I also works with fieldsets.
A cck fieldgroup can be rendered alone. We had some quicktabs we wanted to fill with our field groups. So we've builded a block for each field group to put in the quicktabs. Here is an example of one block where we called a single fieldgroup and rendered it.
How to add another formatter to a cck field? A cck formatter is basically a theming function where the value of field is ran through. Formatters are available at "admin/content/node-type/[your node type]/display" in the cck interface.