Range of values
The range of values of an area of the ROM is which values will work in that area. Often, all possible values will work. That is, most of the time, a byte could be set anywhere between 0x00 and 0xFF inclusive and nothing will go wrong. On the other hand, for example, a two-byte value might be used to index a 4000 element table. In that case, only values 0-3999 would be valid, and other values may cause the game to crash or act weirdly. Any table element marked to be limited in its range of values should be accompanied with an explaination as to the reason for the limitation and possibly documentation of how the game reacts to values outside of that range.