High is 24,000 AGL and Low is < than 24,000 AGL.
VHF is most civilian air traffic, UHF is used in and around military airstrips and may have military TRACON controllers calling out control on both bands.
If you are not near an airport with an active tower, TRACON facility, ARTCC, etc. you may need to get a Yagi [directional] antenna on an elevated mast/tower to get a line of sight path to the facility.
I've been retired from flying for a few years now, but for those years I did fly, the high enroute charts began at FL180 (18,00 MSL). This is called positive controlled airspace. The enroute high charts will show the jet airways, high VOR's, RNAV routes, freqs for ARTCC's, etc. The low IFR's show the Victor airways, VOR's (high and low), limits of controlled airspace and several other things. Both high and low IFR charts expire every 56 days.
Generally the handoff between controllers from local to ARTCC (for those aircraft headed towards high altitudes) is done somewhere before passing 10,000'. When the aircraft gets its clearance (prior to launch) they sometimes get an EFC (Expect further clearance). This may mean that the rest of the clearance will come from center (not the local controller). Another important item is that the aircraft's altimeter is set from the local pressure setting to 29.92 inches. This make vertical separation easier and safer for fast moving aircraft at those altitudes.
In some metro areas southern California for example, you may hear an additional level of control, such as SOCAL. This is generally due to the high volume of traffic.
In the old days, all these charts (enroutes and approach...including freqs) were all done on paper. It was a
pain staking process to do the updates. Now everything is electronic, much easier. better, more efficient.
Hope his helps.
Best.
