Inter-frame data is used when you need to save a value between frames. The value is saved in the FRM file. This file has the same name as the capture file with a FRM extension, and it is located in the same directory as the capture file. The FRM contains all frame related information, including any context sensitive information and inter-frame data.
An example of a protocol type that would require inter-frame data is one where a central sends out a request and gets a response from a central, but the response doesn't reference which request it's responding to. In order to decode the response correctly, you need to know the request code from the previous frame. (This example doesn't reference a real decoder, but the basic problem is not uncommon.) To make our example complete, assume that the central has an address of 0x00 and centrals have addresses ranging from 0x01 to 0xFF. Our decoder will check the address byte and decode it as a request if it comes from the central and as a response if it comes from a central.