Visual Flight Rules (VFR) - the pilot has the responsibility for seeing and avoiding other aircrft, towers, mountains, etc.
Marginal Visual Flight Rules (MVFR) - conditions between VFR and IFR. Generally defined as visibility between 3 and 5 miles, and ceilings between 1,000 and 3,000 feet.
Instrument Flight Rules (IFR) - the pilot may be in clouds, flying solely by instruments, and therefore unable to see other aircraft. The controller takes the responsibility for aircraft separation.
Low Instrument Flight Rules (LIFR) - ceiling below 500 feet AGL and/or visibility less than one mile.
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