Intra-frame data are actually special RETRIEVE methods that take one parameter: the name the data should be referenced by, which we'll call the "intra-frame data name". This isn't a parameter in the traditional sense of an input to the method, it's more like a variable name. Each field you save must have a unique name. This name is used to tag the data to be saved, and is referenced by both the method that saves the data and the method that retrieves it.
A good example of intra-frame data use is in the IP and HTTP decoders. In the HTTP decoder, we wanted to display the source and destination IP addresses in the Summary pane because we felt it would be helpful to see that information along with the HTTP information. To do this, we need to save the values of the source and destination IP addresses when they are decoded in the IP layer, and then retrieve them for display in the HTTP layer. Intra-frame data is the right choice for this type of situation because we have no need of the IP addresses once the frame has been fully decoded.