`fk5_j2001_with_data` is just an `FK5` object, but its `data` attribute is now not `None` (as opposed to `fk5_J2001_frame`, which has `data` == `None`).  That is, it's no longer a raw frame without any data - it's an actual realized frame object with coordinates.  This was @Cadair's suggestion from above - it basically allows any frame object to be used as a template to create a new frame object, but with different data.  

If that makes sense, how do you think I might change the paragraph to make this clearer?