AVR and external crystals
The low, medium and high frequencies are explained in the datasheet. For ATmega16 this is in Table 4 on page 26:



The fuse setting descriptions come from Atmel and they sometimes contain the MHz ratings but this depends on the processor model.

In general you should read the information about fuse settings in the datasheet since there is a lot of information there that just did not fit into short descriptions shown to you by the fuse calculator (they are more of a quick remainder then a reference).