when the aircraft navigation system is tuned to VHF COM frequency (118-137.990MHz) then COM receiver will able to receive two types of signals one is COM signal and the other is AUX COM signal what is the difference between them ?
That frequency range is not for navigation equipment at all, so I would speculate that it isn't related to the navigation system.
They don't receive two types of signals; they do the exact same thing, but the reason someone may want two radios is to make things easier during flight.
What I think you are seeing is your primary communications radio system, and then a switch for an additional, optional "Com 2" unit that may or may not be installed in the aircraft.
Let's say you are talking to air traffic control, and you want to listen to an automated weather station without having to leave the ATC frequency; most systems provide you with an option to listen to both Com 1 and Com 2 at the same time, while allowing you to transmit on only one of them. That way, you could monitor and talk on ATC's frequency on Com 1 while getting a weather update on Com 2.
Another advantage is that you can transition between frequencies faster by simply changing from Com 1 to Com 2 (ie. "AUX COM") after having already set the frequency you want to switch to in the other Com.
Yet another advantage is that one com will act as a backup to the other if either one stops working for some reason.
This may be in reference to the precision of band devision used in radio communications. Most older aircraft radios are only tunable to the 0.00MHz frequencies, while some newer ones may be sensitive enough to tune into 0.000MHz bands. Generally when tuned to say 120.00 on an older radio you will be able to pick up 120.00 as well as 120.005 while some planes are now capable of discreetly tuning to these frequencies. Thus AUX COM signals are those that live in the .000 bands